Well, a thug helping a thug...I don't see it. I think they'd fight eachother because maybe it's their territory or something. =P
And a rogue helping a rat? In the sewers I saw a rogue kill a rat once. =P
You're hitting on what's called a 'social' mob, which is different from an 'aggresive' mob, though the two are often combined. 'Social' is a MUDish term meaning that mobs that are attacked bring others along with them.
There are a lot of variations: only social mobs of the same type, only social mobs of different types, social all mobs, don't social mobs at all &ct.
Monster type mobs can be social for a variety of reasons: they're pack animals, they're intelligent and see a source of something they 'desire', they're peaceful until disturbed...
Humanoid type mobs can also be social for a variety of reasons: they're on the same 'side' (all rogues in a city are in the same 'guild'), they're opportunistic (that player just attacked a rat, if he dies, I can 'get his stuff', so I'll attack too), they're easily perturbed and ornery...
Social-ness is an attribute of a mob just like aggressiveness and part of learning the game is figuring out which mobs are going to try and bring 'friends' along when you attack them.