I just bought a Back 6.5 AL mouthpiece, and its great. It feels very comfortable when i play it and helps me a lot with my problems with the lower register. However, I play jazz, and I'm not sure if this was a good choice, as most people say that a 7c of 12c or something smaller is better for jazz. Was this a good choice?

ps. I have a small bore trombone

12C is as small as you want to go. Anything smaller is overkill, and you won't get a solid tone in the low range. But I know of many people who are happy with a Bach 6.5AL for jazz. There are several professionals who play that size and even larger. Just go with what you're comfortable with. If you're playing lead, you might want to consider a 7C or 11C for a brighter sound and more support in the high register, but for 2nd or 3rd, a 6.5AL is just fine.

I think it is fine to have the 6.5 but i would advise you to use a 4,4G or 5G . I've tried all of them . All are good in high or low register . 4 would give a dark and richer tone compared to 5G . The 5G gives a brilliant sound for any trombone . It's your choice .

I have said this before, and I will repeat it here because I think it applies.
Choose the rim size that is comfortable for YOU. Your facial/dental structure is unique to you. (Personally, I use a very large rim (somewhere between a Bach 3 and 4) for everything, even alto. But I use different cups and backbores.)
From there, if you are looking for a tighter, big band section or lead sound, consider a shallower cup and tigher backbore.
If you're playing more combo stuff, and are looking for a bit more darkness to your sound, a deeper cup and more open backbore would be the ticket.
In all reality, depending on what stage of development you are in, the 6 1/2 AL is a good all around mouthpiece, and you may not even be at a point where you need to get lost in the mouthpiece maze just yet.
At the end of the day, mouthpiece selection is a very personal thing that is best assisted by a good teacher.

my school gave my a jupiter 12 c
i think it has the smalest backbore of all the mouthpieces
i would like to learn more