no, wouldn't put an angel in a 20g long as there would not be anough heght for the fish to freely swim
35g hex is probably about the same footprint as a 20g long, so it is probably pushing it a bit, however the hex has the height advantage over the long....there are probably better options for a centrepiece fish for this tank admittedly, however am trying to find the best option for the guy....theres no point in dismising out of hand when there is a solution which may work, probably at a push