I was just thinking (and please don't kill me), that it's not really fair to give my fish a home and then kick them out when I don't feel like having them anymore. If I keep the schools that I have now, how much can I add onto them without overstocking?
While this is a worthy approach and one I wish more aquarists believed, you do still have to consider that this is your aquarium and the fish you put in it now should live a decade, some much longer. Will you be satisfied with leaving it (with a few more of the two present species) for 10-15 years? Many of us go down the additional tank road at this point, it is one way to deal with the question--just add another tank for "x", then another tank for "y", etc.
As I said previously, you could add to the two species now, and with regular maintenance (weekly substantial water change, substrate and filter cleaning frequently, not overfeeding, and live plants especially floating species) there is no reason this would not work. You are in this for the long term though, and if this does not seem viable it is better to re-home the fish now than later.