roadbed and ballast

I think you are actually talking about road bed as well as ballast. In the prototype the road bed is made of ballast of course (and other specifics underneath) but on a model it is typically 2 separate things. The sub road bed is laid underneath the track, then ballast is applied over and around the rail ties.

Most people use cork for the road bed. There are a couple of reasons for this. Cork absorbs some of the track noise. Of the shelf cork roadbeds are sold with a nice profile which elevates the rail line above the surrounding scenery which looks nice and matches the prototypical practice for main lines. Personally, my layout is as crowded urban switching area where even the mainline is essentially at ground level, so I do not use any cork. One advantage of not using a roadbed is that you can lay out your track right on the "tabletop" and you have leeway to adjust and move things around a bit before committing to the nail.

After all your track is laid, look on the scenery section of this forum for ballasting tips.

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Uni-track

The main problem with joiners is they tend to be a loose fit at the get go and get worse over time. Uni-track, on the other hand, fits together with specially designed joiners and the track roadbed has tabs that stick the track together rather snuggly. These features eliminate the loose joiner problem noted above. However, taking the track apart too many times seems to, in my experience, hurt the track's ability to keep the fitting as snug as when brand new. SO, it's in your best interest to be as sure as possible about the layout before attaching all the track together.

OK, I was thinking of doing

OK, I was thinking of doing sections of 4' in length x 16" wide. I can get that at Home depot in 97" sections. I could get them to cut them in half and then I could make a V or T or Z shaped key hold on either side sort of like how puzzle pieces fit together to hold the sections together. Is 16" enough space or should I go larger? finding pre cut 18" x 4' sections will be a little difficult. 24" will be too large right?
5/8" MDF with a piece of 1/2" pink foam on top for the base. Now, securing the foam to the MDF. What do you recommend and I would probably use those tiny nails to secure the track in the layout initially. What to use to semi secure it semi permanently just in case I want to change it later?

questions on how to secure foam to plywood

ok, so I cut 3 pieces of 3/8" 18" x 4' plywood for my layout. It was free. What is the proper way to glue the pink foam insulation to it securely?
Silicone? Liquid nails? hot glue? The plywood is fairly dry so it will absorb quite allot of glue.
What does everyone use? And to secure the track to the foam? I was going to use cork painted grey or black to simulate ballast. use those tiny nails as well?

Thanks.

Foam Tack

I used Woodland Scenics Foam Tack. Put plenty of weight on top until it dries.

The small track  nails may not hold tight enough in foam. I used 2" foam and found that the tiny track nails allowed some play and track motion so I replaced most of them with 1 1/2" sewing pins. It is much tighter now. In your case, that would not work but you could use silicone latex caulk to attach the track, which allows it to be taken up and reused later on if necessary.

" What is the proper way to glue the pink foam insulation"?

 They make foam adhesive in cartridges, look at lumber yards.       The track can be glued to the foam with caulking , first glue a layer of cork roadbed then the track. No nails necessary, just smooth on a thin layer of caulk so it don't come up thru the ties then press the track in place and add some temporary weight if necessary till it dries.  .DaveBranum