Can I, on 12-2 wire circuit protected by a 20 amp arc fault detector breaker, mix a 15amp GFCI outlet at the beginning of the run, and 20 amp outlets or the reverse, 20 amp outlets with 15 amp GFCI?

You may use 15 amp receptacles on a 20 amp circuit, but not 20 amp receptacles on a 15 amp circuit.