you could probably fit over 1000 juvies in there and probably over 100 6"ers...it wouldn't be purty but they'd fit
haha kidding aside, I don't see any problem housing 4 of any species in a 125g for a year but that's just my opinion. As long as you get them small 2-3" of course. keep up with water changes and you should be golden. I've had a few cichla for over a year now and they would all fit in a 125g.
Just my opinion of course! for a 300g final tank, i'd stay away from the big boys (temensis and pinima).