You can use any kind of diaper you want with them - PF's, fitteds, heck, even AIOs that don't want to keep the leaks in anymore - it matters not.
What DOES matter is that your diaper is absorbent enough. A wool soaker isn't meant to be *waterproof* - the function of a soaker is to wick moisture away for evaporation. If the diaper you use under your soaker can be peed through super-fast, then a wool cover isn't going to help matters one bit. The diaper should be absorbent enough to distribute the wetness slowly & evenly, so the wool can effectively absorb the moisture & wick it away for evaporation slow enough so nothing on the outside gets wet.
So, if you've got a good diaper to use under your soaker, you'll probably LOVE your soakers!
One of these days, Kendell from Freshies! is gonna get her crochet wrap pattern out for us all to play with. But she's in the middle of a cross-country move right now, so we aren't gonna pester her.
I do think someone posted a crochet wrap pattern in the files section of the wool_soaker_group yahoo e-mail list - you might want to go see if it's still there.