For instance, on my Rapid Shot I have it set to use the power up Ability, so does that mean as long as I have RP, it will ALWAYS use that Ability? Is there something special I have to do to use it other than pressing the key on my keyboard quickly to fire the skill?

You have to hold the skill button in order for the RP skill to activate. The RP skill will come up on your screen and your character should have a bluish power up border around him/her when activating RP.

Yes, unless you change t to another rp ability, or reset it, It will do the power up ability. And if you hold down the key, it will automatically use up an rp bubble and add the ability.
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I don't quite understand this, I'm holding down the one key (For my rapid shot) and nothing is popping up, so does it just use it automatically if I set it in the skill menu?

It doesn't seem to use any RP though...Hmm...

Oh! I got it! You just need to HOLD the key if it's set to non-manual, thanks anyways, guys!