i like basoons and i wanted to learn it. my school has a bassoon i could use. i was wondering if playing bassoon would somehow mess up my mouth for playing horn because i want to keep horn as my main instrument.

No.
Bassoon shouldn't mess you up on Horn.
I know 2 professioanl bassoonist who play horn for fun.
They sound good on both.

And I believe the embouchre for bassoon will actually help your low register attacks for horn.

would i be able to play bassoon when i have braces?

Yes.
You should be able to play horn and bassoon when you have braces.
When you first get your braces on you will have difficulty getting the lips over to seal like you are used to. However, this is a simple exercise that should help with your adjustment and get you back to normal in 4 days.

"i want to play bassoon"

don't blame you there, bassoon is a wonderful instrument!

You shouldn't have a problem as long as you practice both instuments. I play bassoon for concert and tuba for marching band, and I have braces. When I got braces it was a little hard to play bassoon but I was fine after like a week. But I didn't start tuba until after I got braces. But yeah, go for it. I love the sound of the horn. Definitely keep playing that.

You'll be fine. Trust me. I've been playing oboe (which is VERY similar to the bassoon, actually, all a bassoon is is like a bass oboe kind of) for 5 years and had to switch to horn for marching band. I played the oboe again, and I was fine. It doesn't mess up your tone, and the embrochure is somewhat similar. It'll just take a little adjusting to, is all.