Incompatibility due to a different id range or due to a more recent date of the core files?!

And what is the range for creatures from other collections – is it custom-made, or is there a link?!

1) Because new creatures were assigned IDs that matched the IDs of creatures from the Megapack (but I didn't even want to include these creatures in this collection).
2) There are no other collections. The creatures are mainly taken from mods that replace creatures in cities and are converted into neutrals with assigned IDs (i.e., they didn't have IDs).
If you look at my list carefully, you will understand the pattern of ID assignment.
There were several (weak) ideas on the forum to update the Megapack, which remained just ideas.
I essentially did this, but in my own vision, and also added some elements: 3rd upgrade, composite armies, duels, new visuals).