Welcome to the learning experience.
I can't tell from the pictures, but it looks like your 500 is the same parkerized combo I have. If I think about the money I spent on mine, just to take it all off and go back to the way I bought it, I think I would cry.
I've got a plain Jane 18.5" barrel on mine, in addition to the 28" and 24" rifled barrel. It is super light with the pinned forend and 18.5" barrel. I was shocked the first time I put it on. If your gun is parkerized, the matte blued 18.5" barrel is a real close match for the parkerization. From a distance you can't really tell the two apart. I agree with the others, though, about just getting a 590. With the money I wasted on my 500 I could have got a 590. Don't get me wrong, I love my 500, but like I said I wanna cry.
I would suggest keeping the mods to a minimum on your 500, and save up for an actual 590 configured the way you want. If you want to put a shorter barrel on, the place I found with the best price on 18.5" barrels is Grab A Gun. That's where I got mine. If you already ordered a forend, your best bet would probably be just to order a slide tube from Mossberg directly. I'm guessing whatever forend you ordered will need the 7-5/8" slide tube and nut. Other places like MidwayUSA and Brownells want too much, if they even have it.