Sorry, whats your question? I cant find it. It seems like you want to ask if having a monstrous npc help the party rules legal. To which the answer is yes...mostly.....
It really depends on how you classify "monster". Is something a monster because it's in the bestiary? it's non humaniod? low to no mental stats? lack of sapience or consciousness?
Controlling the thing would again depend on what the "monster" is specifically.
Something with consciousness or sapience- Diplomacy, bluff, intimidate, or ???
Anything else- up to the Gm
What it really comes down to is DM discretion. Does the DM want the party to be able to befriend a monster.
This just sounds like a case of the group wanting a sillier game while you want a more serious game. Neither way is wrong and if you can't handle their playstyle you probably did the right thing by leaving.