Sounds like you're doing a great job. You'll get really attached to them! I love mine. Each one is a little life and they have their own little personalities too
I would just wait another week or even two before adding anymore fish and try and get the ammonia down to zero with these ones. I know it will be tempting to buy more but that way when you add more you'll have already cycled your tank.
Remember to check your nitrite and nitrate too.
Another tip: when you wash your filter - wash it in dirty tank water when you do a PWC
. I would only do this once a week at most to begin with. And DON'T change your filter every month like the manufacturer advices - they want to make sales! Your filter is where your cycle takes place and if you ever do need to change it then cut it in half and put a new one with it so the bacteria transfers over. It's really important. You won't need to worry about that yet but it's important to know. And NEVER wash your filter in tap water!
You sound like you have your head screwed on - you seem knowledgable as you already know about the nitrogen cycle. I learnt the hard way when I first started unfortunately... I followed what the lfs
told me - I have learnt to take what they say with a pinch of salt! But I have happy fish now