I've converted many derailleur geared bikes (70+) to coaster brake single speed and yeah, 27" with a coaster only is not good. The skinny tires will skid a lot and not slow you down quick enough. Fatter tires are fine. Same set up as on countless cruisers, kids bikes, sting rays, etc. I

I run a shop that is about 2 blocks from the city's skate/bmx park. I get kids every summer who's bikes keep throwing the chain. Every one is due to no or bad wrenching on them. Worn out pressed in 1 piece crank bearing cups or trashed rear hub bearings. Yo!, dad, the chain should not be as tight as you can get it. That will wear bearings and chains out very quickly. Kinda ironic that the biggest bearing surface wears out the fastest.