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

Waterfall_13 Is your new angel defined in the creature's xdb file as an upgraded unit or a basic unit?
If it occupies the first slot in the recruitment window, it's most likely defined as a basic unit. If you make it an upgraded unit, it will occupy the 2nd and 3rd slots.


Added 3 minutes ago
More information about how creature types affect their placement in the recruitment window and their dependence on buildings can be found in the quote in my first post in this thread. It contains all the information I was able to gather through trial and error, organized for clarity.


Added 5 minutes ago
Ah, I understand. If you used the Archangel as the base, it should be read as an upgraded creature. However, it's very possible that the game determines whether a creature is upgraded or not based on the "base creature" line and all values set to "unknown," and therefore records it as an unupgraded creature. That's why your angel is placed in the first slot of the recruitment window because it's considered not upgraded.

Try setting your new angel's base creature to the Archangel, or even someone completely different, like a peasant. It might help. After all, it worked for me with the Gremlins. They appeared in the 2nd and 3rd recruitment slots, so it's possible to do it that way.


Added 8 minutes ago
There's also a tricky line in the creature's xdb file called "pair creature" or something similar.
It's supposed to affect which creature can appear as an additional unit when fighting a particular creature or neutral unit.
But who knows? Maybe in this case, this line also affects something. Although that's unlikely.