I can't tell from the brief description what the "black" beetles may be, but one common pest in poorly stored pet foods is the Black Carpet Beetle, so I'll go with that one. The red legged ham beetles are interesting insects in that they are, for the most part, carnivores. They generally feed on the larvae of other insects, typical of the many species in this family Cleridae, so it is possible they could be in this store in response to the presence of OTHER insect infestations. However, these beetles also feed on the bits of leftover meat on carcasses, so perhaps in a pet store there could be some kinds of "chew" sticks or other materials like dried hog ears or feet or whatever the heck they sell. Due to the very high protein nature of many dry pet foods these beetles also may infest those. I once saw them in a taxidermy shop where they were feeding within the antlers of deer where soft tissues were still present. 

But, like just about any stored food pest the key to control is to find the infested product, isolate or dispose of it, and do a mop-up operation around that area. Then, better storage of the susceptible products and better stock turnover will help to prevent it for the future. These beetles thrive best in warm and humid conditions, so finding a way to reduce both will be a big help. Perhaps it is time for this store to do a thorough spring cleaning, removing all foods from shelves and pallets, vacuuming carefully under and around those areas, and then YOU applying a labeled residual insecticide in cracks and crevices before the packages are replaced. 

You may use insect traps baited with kairomones (food attractants) to narrow the search for the infested products, as well as that good visual inspection using a flashlight and a lot of help from store employees who need to cooperate with you. Look for spilled product, torn packages, crawling insects, etc. The adult insects can fly, so they may occur anywhere in the store. If the black beetles turn out to be Black Carpet beetle they too do best in older product that has been sitting too long, and may gain their foothold in spilled product or damaged packages. Control essentially is the same, that you must find the infested packages and dispose of them and then do thorough cleaning of the area and apply labeled residual insecticides as needed in crevices nearby where the larvae may still be hanging out.