I actually painted this whole lot up a few years ago, put a brown wash on them in 2010, but at the end of last year decided to base them up a bit better, in line with the large bases I started using with my Russians and Germans, and the 3 figure stands I've used for my Japanese and Aussies. For a lot of them I've, stuck with a plain rough sand basing, as they might be actually making a beach landing, but I've also used some static and sprigs of 'jungle scenery' (ie. plastic acquarium plant) on some bases.
The figures are all Revell 'US Marines', and a box of them has given me a Rapid Fire battalion. That's less than my Japanese, for example, and the set doesn't include heavy weapons, but I have lots and lots of Esci US Infantry in various stages of being painted, too, and these are close match (rather than Revel US Infantry, which are produced very much for the Ardennes) and have lots of heavy weapons included, so it would not be too much of an effort to add a few more battalions. Similarly, kit I've bought for the European theatre (eg. a Sherman or two) can be added, although it would be nice to add a few Buffalos or DUKWs. The old Airfix US Marines set was pretty good, too, so somewhere along the line they may get added. In any case, as soon as the WW2 Japanese are completely done, they'll have not just one but two armies to face!
I've found one Rapid Fire Scenario based early in the war in the Philipines, and wasn't sure how these were uniformed until I was watching an old black and white war movie on attack boats in the Philipines, which showed US troops equipped with the sort of helmets more associated with the British, in fact as per US soldiers in WW1, so Hat's WW1 helmetted US infantry might be the best to use. Local forces seem less formally dressed with floppy hats etc. so need to do a bit more digging for suitable figures, maybe the GerMan Spanish-American War Cuban Infantry might even be the best for the part.
These can, of course be used for Korea, too, and even slipped in to Vietnam (in the early part of the war not all forces were issued with M16s).