Sliders come in two configurations, hung from the top and rollers on the bottom.
Install the jamb and give the door vendor the finished de- mentions.

As snoonby stated there are different types of hardware which will affect the setup size. If you purchasing from a millwork supply house or a real lumberyard by telling them what your rough opening is they should be able to tell you and order the correct size unit. If your purchasing these form a box store (HD, Lowes, etc) not so much.

These are from HD, since they have 15% off custom doors right now. Top track supports the weight, bottom track is a guide. They suggested (and I read on a couple other forums to...) carpet first and install the bottom track over the carpet. That seems strange, and means I can't order until I know how thick our carpet will be. Also the track will be smushed down on top of the carpet pile? That doesn't seem nearly as solid as putting down a permanent filler strip of plywood, screwing the track down to that and carpeting up to the strip then starting again on the closet side with a fresh edge of carpet.

I always thought doors first then carpet... but in the case of slider rails it's the opposite?

Generally, top-hung sliders will only have a seperater located in the center of the opening to keep the doors from rubbing together and/or deflecting, either in or out, not a complete bottom track.
There are a number of mounting configurations for this and they can involve a wood block mounted to the floor, to specifically mfg. hdw that stands the guide above the carpet.

If in fact there is a bottom track, finish the jamb, order the doors, from those finished dimensions, install them and I'll guarantee the carpet installers will work around them.

Any special order from a big-box will eat-up any discount.And yes they offer them.

In my opinion special order millwork and a box store equals a sure recipe for disaster but............