Higher level rooms cause stronger disasters within them. For fully upgraded rooms, no matter the size, you should really be using level 20+ dwellers with 10+ damage weapons, and fill that room with them. You will still need to use at least 1 stim per dweller. If your dwellers have very low endurance, they should be trained higher or sent out to die.
If you send them out exploring with some luck, you'll start getting much better weapons after 36 hours (1 day 12 hours)
I've heard that deathclaws will start to show up at 60 population. One thing I did was to send a bunch of new dwellers to the wasteland to die. Just leave them there since they count towards your population. After you hit the population you need to unlock a room (or all of them), dismiss your dead dwellers and you population drops down again. Endurance gear is good for levelling up your dwellers and radiation resistance. After they are level 50 (if they already have high endurance) it only helps in decreased radiation. If you have 11+ endurance, you won't get any radiation in the wasteland, so I would send that level 50 out with +luck gear to have a better chance of finding rares.
You could also stop leveling your dwellers. The creatures (and other disasters) are buffed automatically to match the average level in your Vault.