Is it possible to increase the number of creature slots from 7 to 10 in castles/for heroes?

I think you should define your new angel in the building as the second, not the first creature.
Maybe then it will appear in the second slot. But that's not a guarantee.

In any case, with just one new creature, it doesn't look very good. If there are two upgrades, then I'd like to see two new upgrades as well.
Ideally, just take the core of NCF and add as many creatures as you want. Or, if it's the vanilla version, there's still the Yeti.

Added 3 minutes ago

Well, at least this problem has been solved.

Added 5 minutes ago
It would be great if this could be found. It would be possible to fix the last bug.

I want to increase the replayability of the endgame, as it is done in Blade of Armageddon, thanks to high-level creatures; in that game, dragons (azure, crystal, rusty, magic) and other units play this role.

For the castle, at least, I planned to make an upgrade to the archer building after level 30 of the city, where it would be possible to upgrade crossbowmen to royal crossbowmen (I would add a second shot, like in Heroes 3 for crossbows, although I would have to think a little about nerfing the stats to compensate for the second shot), and archers to snipers (I am upset by the fact that the archers' unique feature, shooting over obstacles, is practically not used in the game, as everyone tries to switch to crossbowmen as quickly as possible, or even to archers; snipers could be made from the "FEM" version of the archer, the red crossbowman with a bow, adding shooting over obstacles without a penalty to the shooting). Of course, I will need to think about the stats, cost, strength, and experience for them.

I would also like to add a branching path for the elves' Ents, with the choice between Enhanced Ents and Dendroids.
I wanted to replace the Yeti with Greater Archdevils, but I haven't started yet, as I'm busy with other things.

There are many good ideas, but without NCF, I won't be able to do it, and I need NCF to work together with PEST.

In short, I wrote to Gerter about my questions and what I specifically need. He said that in the future, he will release a new version of his MCCS, where he will separate what he has already done from the core. Then I will be able to make my own changes on the clean, combined frameworks (NCF and PEST), which will work in Multiplayer (and if the new artifacts also work properly, that would be great). So I'm waiting for the new version of MCCS =)

P.S. I tried to move the upgrade in the Altar_of_Heaven3 building to position 2, but it didn't work; the Greater Archdevil still replaces the regular Archdevil.