I've played Tuba, Trombone, Trumpet, Viola, Bassoon, Clarinet, Bass guitar, and a bit of baritone so i'm pretty experienced when it comes to starting instruments but I don't feel like starting another instrument just cause.

The reasons I like French horn because you always know when you hear a french horn and for the most part they play the melody or something really unique. I have played a marching french horn and I wasn't that good but I wasn't horrible either.

Now if you think I should start french horn, theirs another problem I need to deal with. I'm the only tuba player at my school (upperclassmen wise) and he might not let me rent a French Horn from the school because I might become more interested in that (I'm not to fond of tuba to being with but..eh...)

Other than that, do you have any tips to help me start off on F horn.

I do not think you should take up french horn at this time.

Get to know the tuba better, help out the music group you are in. Without Tuba they are not going to sound good and then you will be in a group that doesn't sound good. So help everyone and be the Tuba player.

I'll still play tuba but in class but would playing french horn at home really be that bad to my tuba playing ability.

not really.
in a perfect world there are some that would say it will affect your embouchre with the different size of mouthpieces and some say french horn takes a different unique embouchre. If you were in a college program, or late high school intending on being a professional hornist or tubist I would encourage you not to do both.

But !! If you are making music for fun and want to learn both (or in your case all instruments)then it will not affect you very much at all.

I don't plan on being a professional tubist. I might love hearing one but I don't love playing one. Right now the only instrument I might go pro on is Viola and horn(if I like)

first off, im just like you, Im always switching instruments just for the fun of learning them and I have played bassoon (they kick a&&, but really hard to play), clarinet, alto clarinet, tenor sax, alto sax, violin, trombone, keyboard, recorder, and flute.
The heck with what that John guy previously mentioned, if it is desire, especially being your age in school, go for it, what could it hurt? Also, you should never be guilted into staying on the tuba! Im really considering buying a piccolo french horn for christmas, just for fun! Hope you decide for the better!! Beetlejuice

I think you should go for it! I don't like to think of it as "switching" but more of adding a new skill to the ones you already have. I play clarinet, saxophone, flute, violin, viola, cello, recorder, piano, and had to play tuba in 7th grade. I personally don't like tuba at all, and I never played french horn, but its sound is rich and mellow. If its what you love, play it!