Tailpice question

I am installing a Gerber overflow/drain on my new tub. The pic. is in the link.My question is it comes with a brass tailpiece that has fine threads on it. I know it can screw up into the T. On the T itself are larger threads where the tailpiece goes in. What are these for? I am planning to go into a plastic P trap. If I use a plastic tailpiece, does a plastic nut and washer go onto these larger threads on the brass T? Or if I use the brass one supplied,do I dope those fine threads and use a plastic washer and nut to connect the other end to the plastic P trap? Does this also get taped or doped?
Also, Is it OK to have the tailpiece/p trap connection lower than where the P traps exit? I am concerned that there will always be water trapped where the joint is, potentially becoming a problem. I had to make my own trap out of elbows that run thru a joist.
It is set up this way because the old tub had a drum trap.The old tailpiece came down along a joist and turned thru it, into the bottom of the drumtrap. The exit of the drum was up at the top of the next joist. I replaced the waste lines with plastic using the same holes they made.This is why I made a P on my own. It has to go thru the joist and up along side it to align with the tubs drain. Hope you understand this. Help and/or suggestions greatly appreciated!

I don't really understand which joint you are concerned about water being trapped at. If it is a glued joint then it should be no problem.

As for the questions on the brass... the finer threads on the brass tube/pipe are for screwing into the tee or drain. The larger threads on the outside of the tee are for metal or plastic compression nuts which would have either a rubber washer or beveled nylon washer. Where brass is screwed into brass these nuts are not necessary and I would only use the pipe thread compound/dope. Teflon tape is the choice of alot of people but on metal to metal I prefer the dope. It acts as a thread sealer as well as a lubricant when assembling parts.

if you are making your own P-trap... is it possible to extend the fixture end with a piece of pvc pipe and trap adapter to be higher than the effluent end?

Here is a pic of the dry fit. Notice that the adapter for the tailpiece on the right, is lower than the bottom edge of the traps exit. This is where the water problem,in my opinion, may be. I cant really add to the adapter side, because when the tub is in place, there is only about 2 inches of space between the bottom of the drain T, and the top of the adapter.
The only thing I can think of is gluing in the tailpiece at the trap, and trimming it so where the tub would actually be lowered onto the tailpiece, then just tighten up the nut and washer to the bottom of the Brass T.??? Sounds half assed,because there would be no way of taking it apart, if needed, later on after its all installed.

drain

You cannot glue the brass tailpiece to the trap. You assemble the entire drain on the tub, (you screw it into the fine threads with a joint compound on the threads. You do not need a slip nut on the outside coarse thread). Then place a slip nut and washer on the tailpiece. Lower the tailpiece into the trap while you set the tub, (or set the tub without the drain and assemble it afterwards), then tighten the slip nut.

If the high end of that trap is higher than the bottom of the tub...the tub will not drain completely. Kinda sorta looks like you need to lower the whole branch to completely solve your problem(s).

Thanks guys. I know you cant glue brass to plastic. I meant to maybe glue a plastic tailpiece to the trap and lower the tub on it using a slip nut and washer on the outside coarse threads of the drain. This is because like I said, with the current set-up , the tailpiece connection on the trap would be lower than the traps exit, thus leaving water always at the threaded connection. If it were glued here,instead, I'd be OK. Also ,Randy,Im trying to use the existing holes used with the original drain,that are drilled in the adjacent joists.Cant drill any others.
Since the tub is in an alcove, what do you think if I raised the tub just high enough to make the trap/tailpiece connection slightly higher than the traps exit?? It would eliminate, the water at the joint, issue. It would only be a couple of inches. Would that look right??

Sometimes ya just gotta do what ya gotta do....a little fancy decor and anything can look really kewl... you would not be the first person to have a raised bath tub....heck, some folks even drop them in a hole and call it a sunken tub....like they are really Waayyyy kewl...