the only consistantly practical use i've found for the down-b is for it's stall properties on the edge. it's few invincibility frames combo with the ledge invicibility and instant sweet spot for some real jerk off time.
If you want to hit with the down-b, use it as your opponent is about to land. So for a combo, I'd say low percent dair to down-b, maybe? Ground will protect you by stuffing attacks meant for you and causing lag from your opponent trying to hit you. You can also do it early to force an air dodge, but they can hit you. Or you can do it late to stuff the start up of almost any attack (besides sheildgrab) or escape route.
I know shears will probably say that it won't work with good players,(he says it in many threads where down-b is mentioned) but the good players actually have to practice punishing this (which only a few will do) before finding a viable way of punishing this strat (especially if you use it rarely). Because you have too many options for them to risk an attack, especially when they are at high percents and you aren't. Besides, who really uses down-b anyway, this is just how I hit with it when I feel like being a jerk...
Laterz...