Can I add one or two more female swordtails to this tank?

Get two or three female swords (best to have one male to 3 females but 2 is okay) and a pearl gourami, you have plenty of room. Wait do you mean a powder blue dwarf gourami or a pearl gourami? Either one should work. I would avoid an opaline gourami unless you get a female as the males can be aggressive, but I think the powder or pearl would be better. Most of your species stay small so three or four more fish should be totally cool especially if you have a good filter and keep the water quality up. You sound like you know what you are doing so I think you will be just fine. Happy fishkeeping.