You could maybe add rasboras, though you'd need a group of at least 5-6 or you could add more black neons.
If it were my tank I'd add more plants before I'd add more fish, some kind of fast growing stem plant like hornwort, rotatla, ludwigia, etc. the sword looks like it's struggling a bit already, needs some root tabs.
Thanks for the suggestions. You're right about the sword struggling. I put one or two roots tabs in a few months ago (although could do with more) and have just starting upping the schedule of liquid fertilizer, so hopefully it will recover. Something else for the back to cover up the mass of white would be very useful!