Have you really got all those fish in there?
Yes I sure do

I feel like my tank is way under stocked lol. How do you keep your sand so clean?
Skimmer, refugium, filter floss every now and then. I vac the substrate when it gets dirty. All in all just keeping on top of things
Are your yellow chromis the ones from IA(Amblyglyphidodon ternatensis
) ?
Yes
how do you like them?
I really like them. They definitely liven up the display. Mine are at a few different sizes so that's kind neat.
I just ordered a few, do they school?
Sorta. I'd say they school about 75% of the time but there is always one away from the rest on watch. It's not always the same fish they take turns. I've read you shouldn't have odd numbers. Not sure why. That's why I got 6 instead of 5.