I've got 6 h. carpinitis (Escondidio) fry that I got a month ago at about 1/2". The biggest 3 (presumably male) are 1 3/4" now, the 2 smallest (probably female) are about 1". So I'd say they grow reasaonably fast if given a lot to eat and nitrates kept super low. Everyone asks this question-basically they'll grow in direct proportion to how much water you're willing to change. Feeding a lot doesn't do any good if you're not willing to do the extra water changes that go along with it.
Edit: Oh yea-a 75 g would be fine for a pair. It's up in the air whether the male will beat the female up or not.
I got 4 from Ken as well. Mine are growing fast (pushing 3" now) but one got killed a month ago and Im afraid I got 3 males left. I know for sure that 2 of them are males. The 3rd is much smaller and slower growing...I hope its a female and not a runt male. Maybe you'll get an extra female and sell me one!