recently in my fish store they have brought in millions of saimese fighting fish (betta splenders) in little jars and i was wondering if i should get one because my only other tank only has room for one.

Here are some questions for it:

can i keep them with my goldies?

do i need a filter?

how many should i get?

what type of water do they need?

do they need a heater?

Blacktail here is a good article with some of the answers you are asking for.
http://www.bettatalk.com/betta_care.htm
In answer to your one question about keeping them with goldies the answer is a no. Goldies are cold water fish 65- 70 degrees or so and betttas need temps from around 72-80.

Also if you raise your goldfish tank temp to accomodate the betta they'll still pick on eachother.

That and the goldies might not tolerate the temp

thanks