Hi , its possible your rabbit could be humping your guinea pigs (hence the mucky backs)as they should'nt be really kept together as rabbits bully guinea pigs, is the rabbit neutered(speyed) as female rabbits can be quite stroppy!!! also i really wouldnt cut the fur with scissors use electric hair clippers i have some small wahl one's which i got from work (im a vet nurse) my g.pig is long haired so i clip him alot, if your still worried ask your local vets to take a look.