A 20 long would be great as well. He may not be eating the guppies due to the fact that they prefer to swim in the top portion of the tank and he may be uncomfortable going up to catch them as he is new. Convicts will head strait down when you introduce them to the tank, so that will be a plus. Although it is possible he has a disease or other illness making him not decide to eat?
going to head to the LFS in the morning and see if they will do anything about it.