Ultra-light trainers might work better than than the avg one's with thicker soles. I've ridden a lot with flip-flops due to extreme difficulty in finding properly-fitting street or cycling shoes. I don't recommend it but OTOH the flexible flip-flops' soles don't cause as much of a problem as I would have thought. Also I read about some runners who use minimalist thin-sole sandals, just enough protection against gravel/pebbles. Still though, I don't really understand why folks w/o fit problems do big rides w/non-bike shoes.