Bit of an important note about the Larvae Swarm, the damage of a Swarm attack is actually specifically based on hit dice. So reducing to 5 RHD would intrinsically mean the Swarm attack would fall to 1d6, but then it also automatically grows to 2d6 with the first class level. Being as how it's damage at end of move with not even so much as an attack roll and brings a Con-based DC against Nauseated, this is fine, being a Swarm is really good action economy.

I'd also consider the adult to still be perfectly playable at 14 RHD, since it does come with a four-attack Pounce and Improved Grab at Huge with +14 Str, and can shrug off pretty much every source of raw-HP danger better than any standard race without serious effort into survivability thanks to how Natural Armor, Fast Healing, and DR/- add up. It's actually a pretty impressive canning of what a PC Martial wants in life, leaving want for item numbers, a Fly speed, and the piles of situational stuff that are the job of full casters readying a laundry-list of Abjurations by scroll, slot, wand, staff, or Contingency. Occasionally Runic Guardian.

...Also the game breaks horribly when you have SR much more than 10 above your level. At 5 RHD, SR 22 is HD+17, meaning it'll take a caster seven levels higher to get a 50/50 chance and a same-level caster has only 15%, and much of what they can do to you will be subject to Evasion and an unusually high Reflex save. The same goes for the Huge adult, including the Reflex save. They be sticky.