Can't really give you a blanket answer for this because it really depends on the size and type of fish. You're probably going to be able to keep 4-6 reasonable size fish in it. 40 Breeders are nice because they have the 18" width which makes aquascaping a lot more forgiving and really allows for a bit more creativity. If you're doing FOWLR you probably aren't going to need a refugium. You can definitely do it if you want but the nutrient export issues in a non-reef tank are much less of a priority. Its an added cost and maintenance task you might want to avoid.I appreciate that info. Whats a rough number of fish that a 40 gallon breeder can hold if running a sump and refuguim? After reading through some of the "starting a saltwater posting" I have determined I will not be doing coral because I want a dog faced puffer.
I don't want to rain on your parade but Dog Face Puffers get pretty big. I've seen them easily reach 8+ inches in a year or two. That size fish isn't really going to work well in a 40 breeder, and the fish you typically keep in that size tank will become a viable food source for the puffer instead of pets.