Guide on adding a new model, animation, and creature to HoMMV

You can simply use XDB to overlay one complete model on top of another as an effect, while making the original model transparent and invisible. You can change the size of this new effect model as much as you want.
But...
If you increase the size instead of decreasing it, the shadows will be displayed incorrectly on the model. That is, you can make it smaller, but it's not a good idea to make it larger. Plus, with this approach, the animation will also not work quite correctly. The timing may be off, and the transitions between one animation and another will be awkward. Plus, you won't be able to apply any effects to the bones on top of this new model. Plus, it's quite a hassle. In short, no matter what you do, there will be problems. It's better not to use this method. Unless you really want to create a small version of a creature from the original Heroes 5 game with the original animations.

Added 3 minutes later

And how do you transfer animations from Heroes to 3ds Max?
UPD: In Max, some textures look different than in the game, is that normal?

Animations. Not possible right now. You need a suitable GR2 plugin, which, as far as I know, has been lost.
Regarding the textures "looking different," you need to clarify the question, how exactly they look different.