you can share as many computers as you want but its going to be slow. if its DCing then i would assume there is a router issue OR collisions are occurring. what are you using to connect your comps? a hub, switch, or router?
so you have a 7 port switch connected to a router that is terminating a fiber internet line? a 256/128 is a crazy slow line... a T1 would be 1500/1500 with a guaranteed up time. you must have some kind of adsl line, there is no way a fiber line would give you such a slow speed. i would not put more than one or two comps on such a slow line. with such a limited amount of bandwidth a token-ring network would be the most dependable, albeit not very practical. i would highly recommend looking into upgrading your internet line. most DSL or cable companies have a business department to provide you with a much faster and more dependable line than a residential customer would get.