I don't see any problem at all with using another butt piece. I know a lot of folks feel that your choices or options are limited with a pre-carved and to a degree, I'd say that's true. You're not going to turn a Bucks Co. pre-carve into a Lancaster school rifle. However, I also think that with some good planning and fore thought, you can do a ton with pre-carves. Round a toe, drop the comb, change the curve of the wrist, add a step, etc...
Personally, I think the wrist area looks a little steep in the pre-carve you linked. I'd play around with dropping the comb (as long as it doesn't get too low...), looking at the angle of the butt piece, which should let you change the wrist/step area, if that's what you're looking for. I see some potential there for some good "Kit Bashing"...
Ed