in a 90, 5 is fine. i had 2 in my 75. and like i said, they bring out shy fish. i call em bait fish. when i bought my queen anthias it hid in the rocks all the time. but after i put the chromis in she would follow them around until i came up to the tank, then hide. he got over that too though. and as far as catching them, i had a harder time catching my percs than i did my clowns. they just swim into a corner and make it easy to net em. VERY VERY peaceful. never had a problem with them.
If the tank has been running that long it is likely cycled, though the only sure way to tell is by doing water tests.