Hiding Chromis

On Wednesday I got our first fish since the tank disaster over a month ago. Went with a blue/green chromis. I wanted only one fish and in a month planned on getting a pair of clowns after the tank has time to adjust. However he has been hiding in the rocks since we got him. I read that chromis are better with groups so last night I got one more to see if that would help bring him out. The new one is very active, swimming all around the tank but the first one still hides. I'm not sure if he has found anything to eat or not since we got him. Is this normal behavior to be hiding that long?
There is no one picking on him. The only other life in the tank is a cleaner shrimp, fire shrimp, and some nass snails.

Honestly they are new to the tank, I wouldnt worry about it, all of my fish were the same way when I got them. Give them time to get used to their new surroundings, it could take a while but they will get over it and come out more

If you are keeping multiple chromis, keep more than 4. The dominant of the 2 chromis will bully the subordinate until it is dead.

Odd numbers are what is recommended, also the more you have in a school the less chance of any one getting singled out and picked on. I have 7 in my 180g and they are constantly schooling and there is minimal bullying. It's also fun to watch the school move around the tank in unison.

Is it safe to add a school of chromis at once? Say 4 total into a 90 gallon tank without the normal wait of 3-4 weeks wait per fish?

Chromis are pretty small so in a 90g you won't have any trouble, add a few more all at once to round out your school. They are better off in groups so I would bet that is why the other one is hiding, he's probably nervous being a loner.