I just want to point out that a rad roach or a mole rat in the waste vs. an unarmed and unarmored scavenger is pretty much 0-1 damage to the vault dweller and a dead roach/rat.

In the vault though they suddenly become fearsome beasts who rip apart rooms full of armed dwellers and multiply if they find an empty room (or make one)

The multiplication ensures that they can get out of hand fast and wipe your entire vault in a matter of minutes if you aren't careful.

They scale with the room ... a level 1 room has very weak roaches and rats ... a level 3 room very strong ... be careful how quickly you upgrade rooms with low level and weak dwellers

Always good to keep at least one person (if not more) in each room packing heat so that you can be prepared.

The one thing I'm noticing with my Vault is a need now to make sure all my rooms are occupied (I left storage rooms and "unused" residence areas empty in the past). If I leave those empty now, the infestations spread pretty quickly.

If this is the case, some warning should be given to the player when upgrading.

And yes, if rad roaches or molerats are in a room that has no one in it for more than 5 seconds, they immediately infest every room adjacent. IMO it is far too fast and creates a massacre if you're not ready for it.

Edit: Also, what constitutes low level? Mid range 30 level people with 12+ damage weapons?