Once Mike learns of the attack on his father, he comes home and notices that some of the men in the house are not Clemenza's people.

Now, Mike's been in the service since 1941 and in college at least a year or so before that. So, how does he have the faintest idea who serves in Clemenza's regime?

Filmic license, most likely. But to give FFC and Puzo their due:
The novel says Vito kept Clemenza close, but gave Tessio freer rein. Michael would have seen a lot of Clemenza before 1941, and probably his people, too. So he'd be looking for them, and not thinking that maybe they retired or were killed in the war since the last time he saw them.

I remember he was at home after his disabling wound for a while, before he went to Dartmouth.

I imagine Mike's familiarity w/ Clemenza's soldiers is due to the 'family' aspect of the story. He would have been familiar w/ them in much the same way that Anthony's best friends were Mike's buttonmen. Its a familia! Madonne! <----

No, no. Even if Clemenza's regime was typical with about 20-30 men, how familiar could he have become with even that few? Remember, Vito had two capos which between them could muster 1,000 soldiers which means each capo had about 500 men to oversee. Mike stayed home for a few weeks before going off to college. So, he's been in the Marines since 1941, then home for a few weeks, then off again. Plenty of time to get to know Clemenza's people, right? Remember, it's not a matter of familarity; he doesn't recognize them.

That works for me.

I figure Tessio and his boys would have been a familiar sight as well.

Thus, Mike would notice if Clemenza or Tessio had a completely new army.

Would there be some tough guys directly responsible to Vito?
Just a thought