Chick starter is a little too high in protein, plus it may be medicated, which should not be fed to adult birds as a daily diet. I would avoid feeding them too much corn... it's like feeding them twinkies, not much nutritional value, just junk food. Small amounts of it given as treats is fine, but if you feed them too much of it they can end up overweight, with all the health and reproduction, fertility and egg binding issues that typically go with being overweight.
Adult Guineas will do fine on any layer feed or all purpose poultry feed with at least 16% protein in it. Pellets seem to work the best as far as food waste goes, IME. If you decide not to feed them a layer feed then I'd be sure to put out a container/feeder of oyster shell for the Guinea Hens, and if they aren't free ranging or have access to small stones in their pen you may need to provide a container of grit as well. They of course always love garden greens, grasses and weeds, fruits and veggie scraps, and I give my birds a flake of fine stem leafy alfalfa hay in every pen (it's good for them, plus helps cure boredom).