A 60 gallon should be fine with 6 angels, along with the bgk. You may end up with a pair, split them up for a couple of weeks, then re-introduce the angel you pulled. Angels like a little warmer temp, 80F is good. I looked into bgk a little, seems the upper temp is 82F, so this should work out.
For show size angels, 10 gallons per fish is recommended. For adult potential breeders, you can get away with 5 gallons per angel. My 65 has 7 adult angels, 2 of which are a pair, another 3 have some strange pairing thing going on. Angels can get pretty brutal when pairing, I had to pull one angel out of the tank who got really tore up by a pair of blacks. The pair doesn't bother any other angels in the tank, only this one.
Just keep an eye on them as they get older, you want to avoid casualties.
Tolak