I am thinking about getting this Mossberg 500

http://www.mossberg.com/product/shotguns-pump-action-500-special-purpose-500-magpul-exclusives/50426

I want the shorter tube because I am hoping that I can find a longer smooth bore barrel I can use to hunt with, as well as a rifled barrel in case I ever want to deer hunt with it.

This says it works with a standard tube, is the 6 shot tube the "standard" tube?

http://www.midwayusa.com/product/742385/mossberg-barrel-mossberg-500-12-gauge-3-28-full-modified-improved-chokes-vent-rib-ported-blue?cm_vc=ProductFinding

I actually purchased a 590 awhile back but decided not to keep it because I quickly realized no one makes longer barrels for it. I am looking for a shotgun that will do everything.

Yep that will work.  Standard is the 6 and extended is the 8 that comes with the 20" barrel HD shotguns.