it would be ok for now - but not ok in the long run. They do grow slow so you do have a time advantage but ultimatley they will need a 125-180gal when they get into adult hood =) I have 2 smaller ones in a 55gal right now and they do fine.
eventually i don't think 60 gallons is really going to be big enough for one full grown adult, let alone four or five. i would suggest either getting a bigger tank or maybe going with tretacephalus instead of the fronts
When I acquired my frontosas they were about 1"-1.5", a year later their size varied from 4" to 6". Rate of growth slows, but continues, after the first year (my largest one is now pushing 8" after 2yrs in my tank).