A thickness sander is the proper tool for making laminations. You can buy one or make one yourself. I made mine from a cheap 4x36" belt sander. There's a bunch of plans floating around to make one yourself, especially from the guys making guitars.
Planers can work. I've used them for simple backed bows and working down boards from 3/4" stock. Dean Torres shows a method where you progressively run the piece through the planer and this works well on most wood. The problem you have with planers is tear out on some species and it becomes unsafe to work with thin stock. The thickness sander is more precise, it's safer, and you won't ruin wood.
As far as a taper rate, I like to shoot for the same as Steve, about 0.006"/inch total. This will give you a nice bend for gluing and should be close to floor tillered off the form if you have a decent width taper. In this case if I have a tri lam, I'll taper the belly, core, and backing each 0.002"/inch for a total of 0.006".
What kind of bow are you trying to build?