Great tank! I wish hornwort stayed that nice in my tank... That I believe is a very healthy amazon sword! Honestly it'll keep growing & get too big for a 10g - you may want to consider trading it in for more plants or swapping it here no the AC marketplace.
Dwarf/pea puffers are freshwater, you may be able to do 2 in your 10g (preferably 1m/1f). I'd definitely add in more plants & branchy wood if you can find it.
My first real "grown up" tank was a 10g. I have to say micro fish are perfect for a 10g. Celestial Pearl Danios, Phoenix & Chili Rasboras are great because you can have shoal of 6-8 easily and still have room for dwarf cory catfish like habrosus or pygmaeus corydoras. I'd recommend 8 dwarf cories and 6-8 of one group of micro fish. They're not tight schoolers like tetras, but they're colorful and social shoaling fishes.
Another option is a male betta. Depending on temperament you could try some dwarf shrimp like red cherries or dwarf cories as mentioned above. Bettas are really curious fish that love to explore all over a tank - people think they're lazy fish but they really enjoy swimming!
Note: Corydoras, puffers & shrimp are especially sensitive to ammonia. As the tank's been set up for a few months I'm assuming your tank is fully cycled and you have done water tests to confirm same?
