Somberlain
Arcane
Started a BG2 run with EE, Ascension installed from BP, and SCS. Simple enough setting right? But there's all kinds of shit, Breach doesn't work on Rakshasa now as it was supposed on the mod setting I had, the game kept throwing short-term prebuffs into mages occasionally though I'd told it not to unless they're teleporting in. Well, I got those fixed by installing an extra fix (link) and then switched to mages never prebuffing short-term spells which got the desired effect. But now there's a bigger fuck-up, the ancient and/or elder vampires in Windspear dungeon keep crashing the game (not instantly but always after a while, must be a specific power they'd want to use).
If I just took back my mistake and got just good, old, unenhanced version of the game, can I expect SCS to work properly there? Or should I use an older version of SCS for that, possibly?
I haven't had any issues you're having with EE so maybe that's not the issue. I haven't played with the new 2.0 patch though, so that might have something to do with it.
Also, some of the pre-buffs SCS casts are invisible, they don't show up in the combat log, so those rakshashas may be using Spell Immunity: Abjuration. Try disabling it and see if Breach works.
Jimfix mod has a component to make those buffs visible.