I find most hypancistrus prefer high protein foods, but will eat a little bit of veg as well (some do, but it depends on the fish), and hypancistrus fry are best fed on veggies and meaty foods.
Edit;
I am not just saying that because the book says so, i just find most of my hypancistrus eat a little veg, a few don't, but my fry certainly like broad beans, plus i have read of a few people who feed some vegetables to hypancistrus, more a supplement with their main diet with high protein foods.
Reading Ingo Seidel's book on L-Catfishes, i was suprised to find in the feeding section that he classies Hypancistrus as Omnivores.
From the feeding section in the book, the omnivore section is just after the Aufwuchs-feeders section, but before the Wood-eaters section and the Carnivore section;
"The Omnivores"
As well as the exclusively Aufwuchs-feeders there are, of course, also numerous genera and species that constitute intermediate forms between the Aufwuchs-feeders and the carnivores, as they are fairly easy to feed on both vegtable and animal food and in addition they are not as highly specialised as regards their digestive tract that they cannot cope without problem with an unbalanced diet for a limited period of time.
These genera include, for example, aquarium-hobby favourites such as; Peckolita, Hypancistrus, Ancistomus and Parancistrus.
These catfishs are characterised by a noticeably smaller suckermouth, and - compared to the Aufwuchs-feeders - a somewhat more strongly reduced definition.
Thus they too possess rows of teeth arranged like the teeth of a comb, and these are again bicuspid, but the number of teeth is signficantly smaller.
And thus these fishes also have only a limited function as algae cleaners in the aquarium. This means that there is also unlikely to be any problem with their damaging aquatic plants.
However, the juveniles of the Hypancistrus species perhaps constitute a minor exception to this rule, as they apparently have a greater requirement for vegtable food and will fill this need by eating holes in the leaves of higher aquatic plants.
As well as eating the types of food already mentioned for the Aufwuchs-feeders, omnivores can be feed with any of the usual animal foods that are offered to loricariid catfishes.
These may include, for example, live and frozen mosquito larvae, Daphnia, Cyclops and brine shrimp, as well as; shrimp, mussel and fish flesh.
As well as flake, granules, and tablet foods there nowadays also special manufactured foods for loricariid catfishes in the form of "chips".
As a rule the ominivores will eat all these foods without problem.