@Wisp thanks fr your time. The issues that from time to time arise are mostly script related, such as a summoned fiend becoming hostile toward the AI or a "friendly" Death Knight standing still without doing anything. I was never able to reproduce them myself though.
I have no desire for you to feel limited in your vision of what SR should be like...
Don't worry, the real restrictions come from SCS and the AI, certainly not aTweak.
...but if you like, a heads up when SR make major departures from the conventional game would be nice (things like removing protection from SUMMONED_DEMON from Protection from Evil or cannibalising existing spells)
A level 1 spell keeping at bay demon lords is a silly game-mechanic I always despised and considering what SCS does (which I supported) I was happy to make it work the same for players. I admit this isa big deal for your PnP Fiends though. :/
I don't "cannibalize" spells that often, and when I do I make sure that the AI can use the new spell as if it was the previous one - standard and SCS AI never had nor will ever have an issue with this aspect of SR - but I'm here exactly because I'd like to extend that check to aTweak AI.
Summons
In general, aTweaks overwrites SR. There are probably a few spell descriptions aTweaks needs to fix. I'll have a look.
I fear this can disturb the overall consistency a bit more than just leaving summons to SR (especially if you alter both cres and spls), but if you think it's the best solution I'd like to see if we can find a way to make it not "stand out" too much.
For example, what do you do with Animal Summoning spells? As I said, our creatures are almost identical, but SR doesn't randomize them (e.g. ASIII always summon wolves) and SR now has an AS spell for each spell lvl from 1st to 7th. I also did the same for Monster Summoning spells.
Fiends
I forget what SR does with Cacofiend, but I'm pretty sure aTweaks is already modding it back to a fiend-summoning spell.
It's still a "gate" spell but I changed the school from Conjuration to Necromancy, and switched the Nabassu with a Death Knight (new cre file, I'm not overwriting the demnabsu one).
When I recode PnP Fiends to better allow it, I'll include an option of not altering Gate (Cleric) and thus preserving whatever changes SR makes.
That would be great.
Even More Revised Fiend Summoning
While I was documenting myself to eventually re-introduce the Nabassu I discovered I don't fully agree with what you did with these spells. I see you added an option to not tie the summoned fiend to caster's alignment which is good imo (why would a demon care about your alignment?) but finding the equivalent power lvl of a devil among demons or vice-versa is almost impossible.
The only easy pick is for the "top players" but even Balor and Pit Fiend aren't perfectly on the same power lvl imo (pit fiend has a bit more tools but less sheer power). On this level the difference is very small though, and thus Gate is more or less fine.
The Ice Devil in theory is the second in command among devils, thus the equivalent of the Marilith, while the Glabrezu is a few steps below among demons (the equivalent devil of a Bone Fiend). Things get even worse when it comes to power lvl imo, because the Gelugon (aka ce Devil) seem to be inferior to a Glabrezu within AD&D (later editions changed this), not to mention a Marilith.
Similarly, the Nabassu is quite superior to a Cornugon because the former special abilities (Aura of paralysis, Death Gaze, Animate Dead, Silence, Vampiric touch, energy drain on hit etc.) are waaay more threatening than the latter's ones (Aura of Fear, Lighting Bolts and chance to stun on hit).
What's your opinion on this matter? Add to it Yugoloths and it becomes even harder to balance each summon with each other.