Unwanted gaps in foam can be taken care of easily in one of two ways (I am sure there are more, but I will cover the two I use). The first is to start of by having clean cuts, and making sure that the parts align before gluing together. For gluing, I use several methods... all depending on what I am doing. Yes, hot glue is a common adhesive, but I have found myself using it less and less. It still has its uses, but I mainly rely on Barge Cement and superglue. I use the superglue for small applications and to seam very small gaps. I use the Barge Cement for almost everything else. The stuff is amazing, and until I tried it myself, I never thought it could do what it does. Contact cement is okay, but over time it will harden and become brittle. Barge stays rubbery forever (which is what you want). If you have done the first step and made sure the cuts are clean and align, when you carefully glue them together, there should be no seam at all.
However, if you do get a seam, the other method is to use a PAINTABLE latex caulk to fill the gap. If it is large, don't try to fill it all at once, but fill it with several applications. Using a wetted finger to smooth the caulking really cleans it up. As Ashuraa stated, it is not sandable, so you want to make sure the surface is smooth before it dries. Try to use as little water as possible, and don't let it "pool" up. You can lightly dab it with toilet paper if you get it too wet.
As far as Plasti-dip goes, it will depend on how much you prep your foam. I always spray a couple of very light coats of polyurethane on my foam before using Plasti-dip. It requires WAY less Plasti-dip this way, as the foam is sealed and does not soak it up. I also live by the rule of only putting on a thin coat of Plasti-dip. If your surface is prepared properly, it really will not take much. For my Master Chief build, I believe I have used around 4 cans, and will need about 1 more can to finish it (it is 95% done).
I hope this info helps!