Start from scratch is a bad way to look at it. You already have a quarantine tank, the most important thing to a new hobbyist!
Here's the thing, just about every fish is territorial. I said a royal gramma or a fire fish could work in a nano tank, and that's generally true. But if you introduce a second fish, they will fight. one fish will kill the other, or one will jump, assuming that a nitrate spike doesn't off them both.
I'm not a fanatic who's going to tell you to go get a 55g. Honestly, you can get a 20g tank and everything you need for under $100, stand, lighting, live rock, etc. a 20g long is like 30" on a side, and you could get so much more for that investment: a clown pair, a goby/shrimp pair, a small school of chromes, a bangaii cardinal, a really colorful dotty back, a small hawkfish, or hell, a combination where you have 2-4 fish! and you already have a small tank that you can place a new purchase in for a couple weeks to detect heath issues.