We have the same size tank and have 8 rays in it, and 5 out if the 8 are between 12" and 16". And the others are between 8" and 12". And they have a ton of room. Not at all crowded. So 6 should be fine, and if I had to chose a ratio I would do 4 females and 2 males.
I guess kinda what I was looking for here was really experiences with male aggression at breeding size with more than one male in a tank that size and with how many females. Should have been more specific. Sorry. Thanks