I don't know jack about kribs but ghost shrimp will clean the hell out of a tank. The more you get, the better they will clean! Don't waste your money on amanos or filter shrimp if you just want janitors for your tank. Ghosties can run as cheap as a dollar a dozen up to 30 cents a piece. I have gaggles of the little freeks - they are fun to watch - and as long as whatever is in the tank with them doesn't eat them, they will eat everything they can minus some algaes. They tolerate a wide variety of water conditions and salinities, but I have seen some of my bigger shrimp catch and eat some cichlid fry, but not routinely or with a great kill%.
I have some ghost shrimp that are a year and a half old (so far) and are pretty big (more than an inch) so if your fish really like shrimp flavored stuff, maybe you need to look at a cat or a cory to do your dirty work. You could always get an infestation of snails - that will clean your tank in a hurry - especially if you're an overfeeder!
I don't use under gravel filters by the way, I just go in and stir the substrate up manually every so often and the ghost shrimp go ga-ga when I do it.
Shrimp are cockroaches in water, come on. They eat crap and smile about it! Well, not filter shrimp, they won't pick anything up, they just filter the water... lazy bass turds
Perhaps you could also consider crayfish?