Crappies eat fish for food, Tilapia eat algae (mostly). The crappie will be taking food away from the LMB (YOY that will grow to be larger food).
IF the goal is to grow large CBLMB, then I wouldn't stock another predator that competes with them.
According to Malone & Sons the hybrid crappies shouldn't be an issue in a pond with bluegill and largemouth bass. In fact recruitment is so poor that they have to be restocked.
http://www.jmmaloneandson.com/hybridcrappie.html
