You are adding about 1/3 more rock and that could cause an
ammonia spike as things in it die off. Presumable you are using new sand and so you won't have denitrification there. I would not add your fish until you are sure you won't have them in a toxic environment. They will be much better off in the 37 with
LR, LS? and a powerhead.
I think the chromis will look fine in a group of 3. Do you have a
sump or is 60 gallons the entire system volume?
