You have several options for low-level vault dwellers.
Explorers
Firstly, you could use them as explorers. Send them into the Wasteland, and see what happens. You never know, they may just happen to find some good items, out there. You can look at your explorers while they are out in the Wasteland, and get feedback on any items they have picked up, too. Recall them, if they find items you would like. They will not take any damage while returning. Otherwise, just leave them out there. Eventually, they will die.
Breeders
You could always use your extra dwellers in a breeding program. In my experience, parent stats do not make a big impact on the child stats. I have had good stats come out of two low-level parents, and I have had mediocre stats come out of high-level and unique dwellers. While charisma has an effect on the success of breeding dwellers, low charisma does not disqualify, either. Even low charisma is reported to work quite quickly. Just keep in mind that the child will count as another dweller, and might grow up to be just another low-level undesirable.
Guards
You could put two of your undesirables in the vault entrance room. Placing dwellers here will put them on guard duty. If the vault is attacked, they will be the first to defend. While you ideally want some higher level guards to defend a more productive vault, low levels make good cannon fodder, and placing high-level guards in the entrance room can be counter-productive. On the rare instance that you have an incident in a room with important dwellers, you could also send your guards in to handle the incident, where you would otherwise be risking the important dwellers. This is likely to be a rare occasion, as incidents can often be more of an annoyance then a risk.
Training
There are many rooms you can build to train a dweller. From 26 dwellers on, you start unlocking the training rooms, starting with the Athletic room. Pop your low levels in here, and build them up on a particular stat. When they have reached the ideal level, you can use them to replace your lowest level, in a corresponding production room. The replaced worker can then be sent to the training room, allowing you a good rotation of upgraded dwellers, while retaining full efficiency in the production room. According to the IGN Wiki, it takes approximately 7 hours to level a dweller from 1 to 5 in agility, using the base agility room. Approximately 3 days will give you a perfect 10.
Best Answer
The cross-hairs with numbers means that the dweller must equip a weapon capable of doing that damage. For example, if the cross-hair number is 14, the dweller may equip a improved sniper rifle 11-14, but not a plama gun 12-13.
And of course, the challenge difficulty is basically told by the required level. If you have few dwellers available for quests, ensure that those with the lowest levels go to the mission with the lowest level requirements. Anyway, since you are limited to three simultaneous quests at most, soon you will have more than enough available dwellers at level 50 for the quests (For quests at level 50, it is better to send dwellers at level 50 and with a high SPECIAL).