Unless you have a 4 or 5 amp BEC built into your ESC's, I would go ahead and use a separate BEC so you can have a decent amount of overhead on the power. You don't want to risk a brownout while flying with the lights on and lose the tri!
There are a number of video's on youtube for tuning. Specific setting are going to be different for you and how you fly and will vary by what equipment you use. I'll have to let one of the other guys post a link since I am not on my normal comp. Having power supply problems with my primary desktop...
I guess what I'm saying is, unless you just want a tad more than what the Rx LED is putting out, run straight 12V. It can be insanely bright if you choose, and it doesn't impact flight time until you really start cranking out the lumens.
Yes, I ran into an issue with the corn lights. The ones I bought had a white domed end like these, not like the ones I linked to above, which had a hollow chamber with the leads bare inside and only about 4mm apart. I had a light crash once and pushed the "corn stalk" up into the chamber which made it short and blow my Rx switch. Now, before I mount them I first bend the leads straight, gently pull the chamber loose from the stalk and fill the chamber with epoxy then push it all together again for safety.
Decided to try the removable lights I made for my quad on my tri since the booms are the same angle and almost the same length. Wanted some light for the rear boom and had a few corn LED lights left. Thought I'd take a few pics showing exactly what I do to avoid the inner wires from possibly shorting.
As I mentioned above, the domed corn stalks have an open chamber with the wires very close and somewhat loose.
I straighten the wires and pull them out and fill the chamber with epoxy
Then reassemble. Those clear clips are venetian blind adjusting pole clips that just happen to fit a corn light perfectly. Had a bunch in a spare parts drawer.