Hi,
... was wondering if it would be ok to add another female to the tank?
yes, that should be ok, BUT:
a male and female cockatoo dwarf cichlid (Apistogramma cacatuoides) for a few weeks
They made their territories! So if you just want to add one more A. cacatuoides, it seems to be a very good idea to take the first two out of the tank for some days before you add a new one. Additional, you can rearrange some of the "furniture" (Sorry, a big lack of english words, it is to early in the morning) of the tank. After you bought the new female, add all three together into the tank, so every fish must find a new area.
In your tank, you can keep even more A. cacatuoides, it is very very important (!), that there is a
minimum amount of males, so the aggressive behaviour of Alpha- and Beta-males is uniformly distributed between six or more male. Following this, you will see a wide variety of behaviour, you will never see with just one pair. Aggressive behaviour becomes more ritualistic and you don't must fear that any fish is harmed. Be aware to put enough caves or other hiding places in the tank - i normally use twoor three caves per pair - and the amount and frequency of water changes must fit the amount of fishes inside the tank.
By example, i keep 12 A. sp. "Papagei" (Parrot?) in a 60 x 60 x 60cm Cube, together with small Hemigrammus and five L134. Or sometimes, when breeding was very successful, i have up to .... *caugh* 60 semi-adult (short before selling them) Apistogramma in 112 litres, water changes of 60-70% every three days. To breed them, i seperate a pair or one male and up to three females, according to the available tanksize and needed fry.
Kind regards
Marc