Hi, and welcome!
Gotta admit, I was nervous opening this thread! Usually when someone asks, "is this too many fish for my tank?" the answer is a resounding yes, with a long list of mismashed, oversized fish that must have the tank bursting at its seams, but this sounds understocked if anything, to me! Not that understocking is bad, it's definitely better to be under than over!
Yes, it definitely doesn't sound like too many to me, even with a large school of celestials.
Whether or not celestials would be okay with a betta, I can't say I'm afraid, I've kept neither.
I'd never heard of danio tinwini before so had to google to see, very nice! Would love to see photos of the tank if you'd be happy to share!