Some can train up SPECIAL stats and others can search the Wasteland. Having a few more Dwellers than can be working for resources at a time can be helpful.

It is worth your time to wait at a certain population level and increase Dwellers SPECIAL stats to increase the efficiency of your resource gathering rooms.

Don't increase your population too fast.

They don't need to hang out in the Living Quarters.

Be sure to put pregnant women and their mate back to work in a room once they are done consumating.

Give those weapons to other Dwellers that can use them. They will always flee when danger is present, so giving them weapons is pointless.

Don't arm pregnant women with any weapons.I have found it useful to send out characters that are not at a high level with a sweet gun and load them up with stimpacks and good armor and allow them to explore until their stimpacks run out, this is particularly useful when you want to lvl and individual quickly and I have found that the cost to revive them ( if you forget to monitor them) is low because they are at a lower level. Overall it is best to have a well rounded special character exploring (as it says in the game) and a higher level character corresponds to more health in general. What has been said about endurance, strength, and luck are true in my experience. I assume that charisma impacts the frequency of encountering non violent scenarios that give you xp ( like when you patch up a run away slaves leg.) Intelligence/perception may also play a role in your ability to defeat harder enemies, like murlocks without taking much damage if any at all. I have a hunch that agility improves your chance of escaping unscathed from a tough battle ( particularly the longer you are out in the wastes).

Fallout shelter gun stats