Oh well i am probably going to get ripped a new one for this but here goes: The 5 gallon per frog rule isn't really a rule but a general guideline and as you get more experience with frogs you'll get a better feel for it. Basically i believe this rule is meant for the small vivariums(ie 5 and 10 gallon tanks). As you get into the larger aquariums/vivariums the rule starts to fuzz a bit. 5 gallons per frog is a good guideline for tincs, azureus(which i think is just a subspecies of tinc), galacs. If your 30 gallon tank is well planted with plants,branches and cork bark that reaches most of the upper part then probably 8-9 adult leucs could be kept and bred comfortably in it.
But it would also depend on the personality of the frogs. I have a female imitator that has killed 5 males(until she found the one she breeds with now) and even in a modified 40 gallon breeder tank(which was the equivalent to a 60 gallons) she tried to kill other females and males when i tried group them. She is perfectly happy in her 18 gallon tank. I would never think of keeping most mantellas in the 5 gallon per frog rule because they would rarely if ever breed. I wouldnt also keep most phyllomedusines at 5 gallons per frog. I know i probably didnt answer your question with all this but my advice would stand that 8-9 leucs should be able to be kept in that tank.But as with all frogs you have to keep an eye on them for the first month or 2 until they settle down.