Not if you keep the filtration up. Every tank is allowed to be overstocked to some extent. My 10g was overstocked because I had 5 platies, 2 mollies, 3 guppies, a male betta, a school of 5 zebra danios, and 6 kuhli loaches. But, everyone worked out fine! I would say that you just need enough filtration, oxygen, and space in the tank. Make sure that no one is too cramped.
I used to have some (3) GoFish that I rescued off Craigslist and they were the coolest little guys. They schooled with the 2 regular zebra danios, and i swear all 5 of them had ADHD. They were off the walls 24/7. I needed up rehoming them because I felt bad for keeping them in a 10gallon tank. I deffinatly wouldn't put any in a 5 gallon, especially not with a betta. They would be darting around too fast and annoy the poor guy. I'm guessing the betta would end up very stressed or the GloFish would end up dead.
Personally 5 gal with a betta is to small. If it was just the glofish, then I don't see a problem. I am biased because my 10 gal had my ct and some glo's. Ombre ended up being so stressed he got Ich. The. He started to nip at the glo's. I originally thought he'd do ok with them because he's so chill.
It all depends on the betta. I had a PK, a CT, and a VT male live happily with zebra danios. I've had females live well with them too. It all depends on the fish. I had half of my 10g divided for fry to go to safety. My male VT never had a problem being in "5 gallons" with the danios, along with a mix of livebearers.
Well, there are things you say that annoy me too, but I try to be polite about it and not mention it. I apologize for my single spelling error. If you would like me to be so immature as to point out every single grammatical error of yours, I would do that. But I won't, since I don't know if English is your first language, or if you speak in a different dialect, or if you had an English teacher that taught the correct way, or if you were raised saying/spelling something a different way. Point taken?
Opinions and facts Kiara... You can't just present opinions as facts and expect them to be correct and accepted. You have anecdotal evidence, which means basically nothing. No offense. Plenty of people have kept their danios in 20g+ tanks with plenty of success.