hi Colin
welcome to the forum
generally garters of a similar size can live together
whether your viv is the right size, depends on the other dimensions
a longer one is preferable, but if you have a tall one, you can create more space with branches
check out this for a lot of good info
http://www.thamnophis.com/caresheets...ake_Care_Sheet
if you're suspicious of getting screwed, you should probably also get some idea of what healthy garters ought to look like . . .