no problem whats so ever. Even the more carnivorous omnivores need vegetables to balance the diet, and the "veggie eating plecs" would be good to get some protein too... basically all plecs are omniovorous- just to varying degrees.
Remember some of the more meat-loving plecos can be a bit more territorial though (like psuedos) so just need to make sure there is enough hiding places for all, and it can be wise to check that certain individuals aren't being outcompeted for food buy more "ballsy" individuals. On the whole no problems though... what you keeping?>