For Glowlights? No, the min is 10 gallons. I think 8-10 is a really good group number to go with! And they really only get nippy if you have like less than 5 in there but otherwise it's nippy toward each other and not other fish like your Betta. So he's safe from them
I find it weird that aqadvisor is saying you can put 20 of them in there, are you adding in your Betta? Also, what are you putting in there for your filtration system?