Unless your assembly came with seals and fittings to make the connection underneath the floor, you'll have a hard time finding an el with straight threads and a gasket to make the seal. As opposed to the more common tapered threads where tape or dope will create a seal, in straight threads, it's more like a hose connection, and you need to compress it against a gasket (which can degrade and eventually leak). Caulk won't help make the lines any more stable. As HJ said, unless you can anchor that thing somewhere higher up, it won't be all that stable. If the floor brackets are to be any help, the pipe and the hole it goes through in the bracket must have essentially no slop, as the stability would only be from those two support points: the el and the bracket. The further apart they are, the better it will be at stability, but you are relying on the stiffness of the pipe to then keep it in position. Pulling on the hand-held, and even just turning the valves on may overwhelm it. You need something to stabilize the top end for maximum reliability. Is there a wall anywhere near that you could maybe find a finished bracket to run to?