no rule, but the most practical approach would be to choose herps with the same husbandry requirements. it's easier to maintain the ideal ambient temp and humidity in one single herp room rather than a multitude of different enclosures that are sitting next to each other.

i want a lot of different boids, heck a lot of different snakes period. but i only have ball pythons at the moment. i do not feel that makes me a specialist. rather, i view it has having a limited collection....

i know only two fellow herpers who intentionally keep their collection to only one particular species (one keeps retics, the other burms). the common thing about them is they are not (yet) interested in other snakes. i think the general personality of herpers is liking all sorts of interesting animals, and diversity of species in collections is the norm. i don't know about you guys, but where i am, the more advanced and experienced a herper is, the more animals he has.