Am I right in assuming that if an ability has a ":" in the pay cost you can use that ability as much as you have resource available such as "Pay 2 credits: Break a subroutine" , and if it doesn't have a ":" you can only pay it once?

Not exactly. Abilities in the format [cost] : [effect] are paid abilities, and are separate in that they can only be used in specific paid ability windows* (see the timing charts for when these windows occur). If the cost includes one or more [click]s then it must be done as one of your Actions for the turn

Cards without this structure will tell you when they can be used, such as the bypass ability on Femme Fatale which can be used whenever the specific trigger condition (in this case, encountering the chosen piece of ICE) is met, but only once PER instance of that condition being met. Some cards (like Imp) may carry additional restrictions, such as once per run or once per turn.

I hope that makes things clearer, and not worse!

(*unless the effect is a Prevent or Avoid effect, in which case you can only use when the effect you want to prevent or avoid is about to resolve. Netrunner timing is complicated, sorry )

Thanks again.