By limited I mean there are not that many to choose from and this means that I might have two people with BEAM Array overload ... or 2 with torp high yield.
Do they have identical cool downs? ... ie does triggering one ... stop you using the second?
They have a "global" cool down. On some powers it is short enough to rotate 2 of them and allowing one power active all the time. This is because the global cool will be shorter than the used cool down. Emergency Power to Shields is one example.
Typically you can have 2 copies of a power which improves your overall usage rate of that power due to their global cooldown. The global cooldown varies, but for tactical powers I believe all of them are 15s.
This means that if you use say tactical team, it goes into cooldown for 30s. But the global cooldown being just 15s means you can use another tactical team when that global cooldown is up, which means 15s after you activated the first.