If you are using the strecch function in FL then you can set each chop to the required length eg you might have a couple that are 8 beats long, a couple that are six and one 4 so you set them accordingly and they will all play at the same tempo, obviously your skill in arranging them will dtermine how choppy or smooth / good or **** they sound. You might also want to look into using the key/choke groups in the sampler pane so that each chop mutes ther next one out, it then doesnt really matter what length they are, you can 'play' them as you like

Also is you are prepping your samples in Audition pressing F8 will drop appropriate loop points in the sample whenever you press it, if you then open it in the Fl slicer it should use these to set the slices rather than the (pretty wack IMHO) algorithm it normally uses