The dead-end wires on individual fixtures (typically 18 or 16 gauge) only need to carry the current for the one light so whatever size they are now will be fine, unless it's one of those real cheapos that is drastically underengineered, built in a sweatshop, and gets recalled in a few months due to a fire or burned customer. You'll be fine. Those lights don't draw much.

My guess for the size ofthefeed wires would be 12 gauge, but without the current or power draw numbers for each light, it's just a guess. Definitely go with the stranded. It's not only easier toinstall, but holds up to vibrations and abuse much better than solid wire.