Forum:New to the xyz mechanism, in need of clarification.

So I recently got back into this game a few days ago and decided I would get myself some new sweet Xyz cards for a deck. But the main problem I have is on summoning them:

1- In the booklet of the deck, it says I can Xyz summon anytime I control the right materials, as opposed to synchros where I can only do it in my main phase. So my question is: Can I do it in my opponent's turn? And if so, what speed is it considered? Can I chain it to the activation of, say a bottomless trap hole? Can it even be used as a response (the xyz summon) to something?

2- Can I still Xyz summon a level 5 Xyz monster such as Tiras, Keeper of Genesis in this situation: I control two level 6 monsters, and have two Level Eater in my graveyard activate their effects, making the two level six monster level five now. Is that allowed? Because I haven't found anywhere that the level required has to be the printed one, but I'm not sure, so that's question.