no, in theory it is possible.
See, how modding works, is that a modded exe(or dll to be specific) forces the game to load stuff in a certain folder OVER the stuff it loads from the stock game.
so, when you launch a modded exe, it will look for such a folder, by a specific name. In the case of "classic modded game" it will look for a MODS folder. For and SAS game it will look for a folder call #SAS, if you select a UP3 game, it will look for a folder called #UP#.
So, you can take your whole #SAS folder for example, and name it MODS. And if you select "Classic mod game" on the selector, it will read the "MODS" file, which now contains SAS mod-act 3 stuff, and so, you will launch mod-act 3 in practice.
Same with UP. If you put your UP files in "MODS", and launch the "Ultra Pack 3" option in the selector, your game will not read anything form any folder called "MODS", only from a folder called #UP#. If your UP stuff is in your MODS folder, it will thus not be read.
To make it be read, you must select "Classic Modded Game", then the game will read content from their MODS folder.
it may seem a bit confusing, but once one has your head around it, it is really straight forward.
My advice is though, that if one is unsure, best not to mix your installs, and to only install things one at a time, and very carefully, so you can remove it easily if it does not work.
But do yourself a favor, if you are serious about making your life in modded IL2 easier. some years ago I was just like you. I did not know anything and even simple things confused me and I got things wrong and messed up many installs, and did not know enough to get out of it.
And back then, at AAA, there was zero information available, and no help forthcoming.
So I took a weekend once, and started just searching and reading, and writing things out as I understood them, until I got a solid basic understanding of more or less how modded file structures work.
Later, when I started this forum, I put all that into tutorials, which are available here. IF one takes some time and patience, and a willingness to learn, you can sit down and go through them carefully, and at the end of it you will understand a whole lot more about the game, and be much more able to help yourself. But I understand, one does not want to do that, one just wants stuff to work.
However, if you do not know enough, then don't make hybrid or customized installs. Take what and install gives and leave it at that. If you have a working install that is highly customized like your one you mentioned, then for Christ sake don't go install something else over that. Make a copy and work on THAT. Make multiple installs so you can experiment.
I know my way around mod BACWARDS. And I can tell you , I NEVER EVER EVER install anything straight into my personal "Ultimate" game install. I have many small installs with simple activators to TEST stuff on. And if I cannot get them to work there, I don't go ahead and install them into my personal game anyway and hope for the best!
If I have a proper modded install of HSFX, then I will have another clean install of HSFX for backup, and yet another clean install of it, for checking out updates and new mods on first.
Same goes for everything else. I have a UP3 customized, I also have a UP3 clean tester, a UP3 backup, and I back up my #UP# folder weekly, so even if I really fuck up, I only step back at most 7 days, to a fully working copy.
I understand that int he flight Sim world, Obsessive Compulsive Disorder is rampant. I too am a sufferer. You WANT all the mods in ONE folder and no other dirty little installs floating around. It must all be neat and tidy and in one place.And you must install all mods and packs and updates into that, and if that does not work, you have to start from scratch every time.
In my opinion, that method is at best completely irresponsible, and at worst, truly insane.
So, perhaps a request to everybody: Don't come tell us that we ruined your working install that you worked years on and now it is lost forever. If you run mods that way, without backups, then all we can really say is "Pity the fool!"
And as I said to Raptor when we talked about this a while ago: The argument about not having space on your harddrive for multiple installs is crap. I bought a 1.5TB Seagate barracuda 3 weeks ago, and it cost a lot less than I paid for my Mouse. If you cannot obtain more drive space in this day and age, then I think a messed up IL2 may be the least of your problems in life.
So, make copies, make copies , make copies.
have copies on backup drives, have copies on your harddrive, have copies of 4.07, of 4.08, of 4.09 and of 4.10 and of 4.101. Have multiple copies of 409 and 4101. Have copies of all your installs.
Have copies you can fuck up and learn from. Have copies to test with. Have copies to play with. Have copies to mod with. Have copies of every working install you have.
NEVER install more than 1 thing at a time before testing. NEVER install anything straight into your MAIN KING PERSONAL CUSTOM SUPER INSTALL (the one you play for fun)
And then, make backups. Make backups of everything, all the time.
With the new method of mod-act we made, it is possible to have a UP3, SAS3 and HSFX5 install all in one super install. But it requires just a little bit of management from the user side. If a user does not know or understand the game structure enough to manage that, then he must please not attempt it. Rather then just have separates.
I have many installs, but I have my one big super install. My personal game I play for fun. And even with all my knowledge and troubleshooting skills, I never ever put an untried and untested mod straight into it. I also never put a mod into it that I don't absolutely want and need. For novelty planes and mods, I try them out on another install, to see if they are truly worth it. I have to say, in my super install, I have never had more than a handful of new planes in there at any one time. Having every mod ever made in one install might satisfy one's OCD, but it is also a surefire ticket to disaster.
My advice about modding is: Be sure, be slow, be methodical, be careful, be conservative, and most of all, the golder slogan of everything that is gold in the world of hacking:
try before you buy
Although everybody does, you should in fact never come to a forum like this and say "I put this into my only copy and it messed it up and I cannot get it out and things working again.
It is ones own responsibility to educate yourself, to test, to have backups and copies, and to, when you do install something into your BIG INSTALL, to carefully note every step you take so you can step back properly if needed.
So, in closing: Take a deep breath, walk away from the game for a few hours/days/weeks (like I do when I get so frustrated), then come back and set yourself up to be able to try things out properly and play with them and learn from them, without having to risk your high-value installs every time you want to do something new.
Best of luck with it! And believe me, the path I describe is worth the effort. The game you will end up with will kick the ass of any other flightsim any day of the week and twice on Sundays. I have had big "ACES" from the big MS and other flightssim "clubs" here, come sit behind my PC to try out this "little outdated Russian simulator", and leave 8 hours later, pale faced and silent.