For female crew members, I'd just go with the female crew sprue - that way you get appropriately scaled female bodies and arms too.
I'd say the best choice of alternative heads comes from the Stargrave mercenaries sprue, which includes various interesting helmets, more aliens (an ape, a racoon-type, two lizards and a 'bum-face' Star Wars alien, among others) and various grizzled-looking humans.
The Stargrave troopers sprue also has helmeted heads (of two main types), plus male and female heads (the bulky armour of the trooper bodies means that body size and shape is less of an issue).
Also, the female crew sprue has some alien heads that could be either sex (or something else).
Buying an individual Stargrave sprue is probably little more expensive than buying a set of heads, once you account for postage, etc.