Here's a long post from me.

First of all, it's really cool that you finally managed to port models with animations into the game. Once again, huge congratulations.
Unfortunately, I don't have time to check it out in the game right now.

I'm especially interested in whether there are any issues with the synchronization of the attack and damage animations. In other words, do creatures with new animations play the damage or attack animation too early or too late?
Is there a pause effect in combat between this creature and a creature from the original game? And if not, how did you adjust the animations from Heroes 7 to match the timing of Heroes 5, or did it just work out naturally?

Regarding the movement animation. I think this is a very easily solvable problem. Take the idle animation from Heroes 7, loop it, and if the original duration isn't enough.
Take the bone responsible for the model's waist (ideally, it should be the parent for the rest of the skeleton), move this bone to the desired distance, create a new keyframe,
then adjust the movement of the walk cycle up and down along the path to this frame (if this wasn't done in the original animation). And basically, you're done.
To better understand the task, it would be helpful to do a little research on the basic principles of walk cycle animation.
In principle, I've just described it in a superficial way, but it's better to get information from a more competent source. It will also be clearer.
If there is an opportunity to import the movement animation of a creature from the original Heroes 5 as a reference for timing and distance, then everything becomes ridiculously simple and can be done in three or four clicks.
(Frankly, the decision of the Nival developers to make the movement animation as a series of jumps is debatable, and on some models, the seams between the "steps" are visible.)

Regarding the models from Heroes 6. Just so you know, many of the models from Heroes 7 are recolored models from Heroes 6, with the original UV mapping preserved. For example, assassins, minotaurs, namtaru, etc.
There are also some models with minor changes.
For example, the medusa has the same geometry and body UV mapping as the coral priestesses, but a different staff.
The staff is a static part of the model, hanging on one bone, and it's very easy to change one for another.
By the way, there's a big advantage to this whole theme of recoloring Heroes 7 - when porting such a model, you immediately get twice as many creatures, because both the textures from Heroes 6 and Heroes 7 fit it.
And there are simply models taken from Heroes 6 without changes, demons, succubi, etc.
In essence, there are very few models from Heroes 6 that are not in Heroes 7 and that cannot be made using the skeleton from Heroes 5.

Regarding models from the world of Ashan, Dark Messiah is at your service. The game is made on Source, and it's quite easy to convert models and animations from it.
And of course, there's also Might and Magic X, with models from Heroes 6. But I don't know a way to get the models from there, and I haven't seen anyone do it. Although it's possible that such an opportunity exists, it's just that everyone is so indifferent to Might and Magic 10 that no one has looked for this opportunity.

Another nuance, ideally, when porting these models, you should do at least some retopology. Or at least some kind of crude, automatic LODs for the global map.
I won't go into details because I'm 99% sure that everyone will ignore this step anyway. I hope there won't be any problems with this in the future.

It's funny, but now, with the ability to easily port animations and models, and with such a large database of resource donors, you look at modding Heroes 5 a little differently.
Here's a question, has anyone wanted to create their own models or animations from scratch? Or is everyone happy with the fact that no high-quality mods with original visual content for Heroes 5 have appeared, are appearing, and, apparently, will not appear?

P.S. Kha, the walking animation on the titan's skeleton. You've been working on it for how long, a year and a half? You can literally take the file archive and copy it to the root of the game, and it will work.
I understand that no one has free time, but I don't understand why, under these conditions, you would continue to work on something that already works without any problems.