Bally - I know that 'cabin service' is an archaic notion, but in the days when that fine airline DanAir ran a superb business class cabin service we found that whilst the 'econ' LGW-CDG sector showed FL300+ for a light 737 (with about 1 min at cruise), stopping at FL240 gave a much easier ride for the c/c for heaving trolleys up and down the mountain. Fuel burns were little affected and there were sometimes routing benefits too in the lower sector.
APU-wise
1) Boeing (737) 'recommend' starting below 250 for better start reliability but as IC says, no limit there.
2) DanAir (because of the way our APU maintenance was costed) found the improvement in 'life' by leaving it running LGW-CDG outweighed the fuel costs by eliminating rapid hot-cycling.