I agree w/ the others, your already overstocked. Best to avoid adding anything new.
when they get bigger its time to upgrade
Fish will not usually literally "outgrow" a tank in the sense that they get physically too large. If fish are not provided w/ the proper amount of room, they will have many issues, that are not reversible, before they start showing signs of stress. Such things as skeletal deformities, and problems w/ internal organs. Things not very easily recognizable by appearance.