2 Nalgenes works fine. Ideally, each guy could get out and put back at least one bottle without help and without taking off his pack. Slightly less good if he needs help. The easier this is, the more likely people are to drink. Many zip pockets, and even some of the side mesh pouches, are difficult to get at with your pack on. If someone does use a hydration system, the MSR version is pretty reliable, and the opening is the same size as a nalgene for use with filters.
With regard to the tents, etc., make sure you keep an eye on group gear distribution during shakedown. The ranger should help with this, but you have a better idea of the relative ability levels of each person. Try to equalize exertion level, not necessarily weight. This is particularly important if you have a variety of ages/fitness levels. Also, remember food is consumed somewhat unevenly, so you can redistribute as you go, or give stronger guys food that will be eaten last. If you have good crew leadership, this is an excellent thing for them to do. The Scouts will have a sense of fairness and also be interested in moving the trek along, particularly if there are one or two significantly slower hikers.
Some shorts links (many on sale at REI for a few more days):
Convertible pants are nice, because they cover the Philmont pant requirement for certain activities (spar pole climbing, horseback riding?) without too much extra weight. When I'm hiking in the summer, I usually just take rain paints.
http://www.rei.com/product/746908
http://www.rei.com/product/762162
http://www.rei.com/product/718896
Longer cargo-style shorts:
http://www.rei.com/product/746923
http://www.rei.com/product/762176
Mountain Hardwear stuff - great design, especially the waist, but a bit pricier and not on sale:
http://www.rei.com/product/755066
http://www.rei.com/product/762268
I actually own these shorts - purchased in Reno after a Khukvention. Very comfortable, but the back pockets aren't much use with a pack on.
http://www.rei.com/product/718899?vcat=REI_SEARCH
I also own these shorts. The cut is better for around-town use (longer inseam, so more fashionably correct), but good for trails too.
Another good deal on M.H. convertible pants:
http://www.sierratradingpost.com/p/...-Hardwear-Pack-Pants-Convertible-For-Men.html
Browsing Sierra Trading Post, Campmor, and the usual outdoor stores (Cabelas, etc.) is worthwhile. Primary requirements are tightly woven synthetic fabric with a loose cut for mobility, and smooth fabric around the hips (no bunching, elastic, belts are a minus but seem to be popular). Having low pockets is nice - back pockets and pockets that sit high up are tough to use with pack on.
Honestly, the basketball-type shorts they are wearing would be OK. The mesh might snag on branches, and might not hold up to lots of abrasion on rocks. Lack of pockets are a minus, but my biggest worry would be getting them sized relatively closely in the waist so there isn't extra fabric bunching up under the hip belt.
Mesh tops are OK. Personally, I like something more like the weave of your average T-shirt. It's not quite as cool and doesn't dry quite as fast, but fends off brush better and is more comfortable in the morning, shade, or in light winds.
Hope you find something useful in all that.