There are no cave-ins, except with an unsupported tile (unsupported = not connected orthogonally to a construction or natural material. Bridges don't support constructions or other tiles.).
Wells must be constructed over more than 7-deep water to avoid the stagnant water complaint. A smoothed or constructed cistern with refilling method is ideal.
Food and cloth are decent, leather and bonecrafts also work to a degree. Also look into pottery or glassmaking if your embark has clay or sand, respectively.
Stocks can be kept inside the hospital zone by building coffers/chests or bags there. Some people report problems with overfilling (triple the set amount of cloth, for example), so I just build the containers one at a time. 6-10 should be plenty. Don't forget soap.
Caverns aren't that big of a deal, if you assign everyone masonry and wall off the stairwells. Might be worse if you've waited a few years, vomit blobs and steel beetles could be lurking.
Archers will practice at archery ranges when idle (not on an assigned training schedule). Melee dwarfs will only do individual combat drills unless they are assigned an active schedule with training orders. on the ( m ) ilitary -> ( a )lert screen, make sure there's a green A next to your melee squad's name when you have the active/training alert selected.
Military size is mostly personal preference. I've gotten by with a squad of 5, but that's using a combined-arms defense of simple traps and soldiers. My current fort has 7 elite melee, 10 reservists training melee, 8 elite crossbowdwarfs for patrolling the inner keep/manning guard posts into the carverns, etc. Keep in mind injuries/deaths do happen, plan squads accordingly.