Is she in the 30 gal right now? If so you need to get her into a bigger tank before she reaches 5". A 55 gallon is perfect for 1 female, unless their aggression levels are extremely low then you could keep 2.
Males get too big for a 55, they need at least a 65. You could probably keep a male & your current female in a 30 gal for about a month. If your female is about 4" now she is either in or about to hit her growth spurt and could be 6" in less than 2 & 1/2 months.
I would not keep 2 females in a 30 gal, like I said before unless both are extremely non-aggressive they will not tolerate each other in a tank that small.
