Welcome to SGW ri2300
Lot's of good folks around here who will love to help you with your (what will soon be) new addiction.
If I get a NEW Remington 870 Express Synthetic 26 or 28 inch long barrel shotgun could I also buy a interchangeable 18 inch long barrel for home defense?
Yes there are many different types of barrels made for the 870 (and with a few exceptions) all will interchange. The 18" HD would run you about another $119.00 from Cabela's
Also what is the difference the between 26 and 28 inch long barrel besides 2 inches?
Depends on your intended use of the barrel. If you want to use it for clays or bird hunting the longer (and a little heavier) barrel will probably give you a smoother swing. But that depends on your physical build too.
Most stores likely stock the 28" version...get your hands on one and see how it feels to you.
Lot's of stores usually run frequent sales on the 870, typically with the 28" barrel. If it's not on sale this week it may be the next.
Also ask your dealer or check the Remington website, there is a rebate promo going now that will get you another $30.00 back. But it ends on 12/31/2008
Hope this helps a little.