No POD that states before 1950's,please.Now,Fellow Althistorians,Think!
Not gonna happen unless it's just by resupplying the Cubans. The US won't allow it. Cuba was troublesome enough. Unless you manage to squeeze it in during the later stages of Vietnam, where the US is perceived as being a bit more impotent than normal.
It also requires the Russians to actually give a fuck about Nicaragua.
But the most soviet troops you might see on the ground might be the odd advisor teaching people how to operate equipment, and provide the odd bit of experience here or there. At most it'd be technicians, some pilots and maybe a couple companies of paratroopers or marines. Any actual deployment of troops in small numbers would have to be really quiet to make it work though.