Hello! Perhaps somebody here can tell me if monsters in BG1 have the maximum amount of HP granted by the game itself ? I have this sensation but I would like to be sure about it. Thanks!

I could be wrong, but I believe that it is based off of the difficulty level you choose. Go to the difficulty selection, and it gives you a description of each setting. That should be it, but I don't remember.

I think they have fixed HP, directly read out from the CRE (creature) files. The amount of damage they do and the experience you get depend on the difficulty.