Peppermint products using alcohol as a carrier are probably fine if they are applied to non-plant surfaces (like the container soil surface, or maybe to something absorbent that can radiate the scent, like a popsicle stick that can be wet-down with the liquid). If the product says it can be used on plants, then it's probably formulated to avoid leaf damage. If it doesn't say that, then we agree it's safer to be cautious and not use it directly on desirable plants.

Squirrel deterrence is challenging and there are no easy, reliable solutions. You could try "mulching" any bare soil in the planters with something prickly, like the fallen seed pods of our native Sweetgum tree, or perhaps pine cones from some of the pine species with smaller cones (they can be quite prickly). That might deter a squirrel from trying to dig and would potentially be easier to put in place than trying to bury chicken wire around desirable plants. You might also try one of the expanded slate products (sold as vole-blocking soil amendment, often) as another material they are less likely to bother trying to dig into. It's not super sharp, but is still stony and an inch (more or less) layer atop the soil would probably make them wander elsewhere.

You could also try a scare device like shiny Mylar strips tied onto a stake that can flap around in the breeze, which probably won't deter bees and butterflies but which might (at least until they get used to it) spook hummingbirds. Putting out a fake owl or snake mannequin might make the squirrels uncomfortable too, but it might need moving around periodically so they don't realize it's not a threat. Garden centers probably sell these products, and may also have balloons that have large fake eyes on them to similarly startle nuisance animals and make them feel hunted by a predator. We don't have research that measures the success rate with any of these approaches, though, but at least they are harmless to other organisms like pollinators.

Miri