So lets see if I have understood this correctly.

"When resisting an Opposed Focus Power Test, a psyker may select a Power Level for any Willpower Tests to resist a power, adding his Psy Rating as normal to the Test."

Does that mean he gets to choose from Fettered, Unfettered and Push and if it does, can he cause Perils of the Warp by resisting ?

I would think that this is the case, yes. Resisting a power from warp using your talent originating from warp should cause it. Although you can hold back and use less psy rating, and get no risk of phenomenae, but get lower chance of success. Also remember that this can actually cause double phenomena/perils if both you and your attacker gets doubles or pushes.

This is just my opinion. Other thoughts?

I love this, and this is how I will play it from now on.