Neons don't always have diseases and can adjust to any PH. It depends on where you get them and on how realiable the source is. I have 12-15 neons in my 20g and never had an outbreak of dieases or anything.I thought about celestial pearl danios with the bettas but I was worried that they would be too shy. Maybe another nano fish like ember tetra or sparrow rasbora or something would be better? Neons, personally, i'd be too worried about them with diseases and getting the pH and what have you right.