You can add some hardy fish like damsels and chromis right now if you wanted, but before you get any expensive fish get that ammonia down to 0
Honestly I think you could put a moderately hardy fish in like a Valentino puffer, but it's better to be safe than sorry... Not fun to learn the hard way.
hope I helped!!
Not having a quarantine tank is like playing Russian Roulette, nobody wins, you can only see who plays the longest... Unfortunately, I don't have the time, money or patience for a quarantine tank so... That sucks