The problem is meeting the bare requirements isn't always the best. Bettas can survive in .5 gallons just fine, but we like to give our bettas room and a big tank, so why skip out on other fish lol. Also, minimum isn't to aim for, you want more than bare because it will make them feel more comfortable and lead them to less likely terrorize your betta, I always recommend 8+ and some other members have agreed with me.
I would add your betta after a week so the neons feel comfortable in the tank and know where to go to escspe.
Chocolatebetta that doesn't make sense, neons aren't as active as zebra danios, and there is two types of 20 gallons, long and high, which is like a 10 gallon stacked on one another, it depends on activity levels.