Well you can hang one HELL of a lot of extra gear off an alice pack with the alice clips or molle attachments.
My medium alice pack on molle frame with molle straps and waist pad setup carries two two quart bladders or two one quart canteens on the side of the pack, E-tool in carrier above the middle pocket(which cannot be loaded very bulky because of the carrier) ecws stuff sack for bivy and green bag or poncho and poncho liner in a carrier below the pack, sleeping pad on top of the pack wrapped in a poncho and held on by the main flap straps, another two quart bladder in the top flap compartment, first aid kit on one side above the canteen. thats all external.
This particular rig is for when I am going to hike in to overnight at rockhounding areas when I dont expect it to be much below freezing. The partial ecws with poncho liner added under a poncho shelter should work down to well below freezing with a space blanket fire reflector and small campfire.
Note its heavy on water, gallon and a half, but this is arizona. If it was my east texas stomping grounds I'd only take half gallon of water cause that is mostly lowlands with lots of streams and ponds.
The large ive not loaded up one in years, but many moons ago I spent 13 days in the smokey mountains on foot with one and a flyrod and came out with food still in the pack.
I've managed to get a large onto the molle frame, but I was not very sastified with how it sat, it seems to be too low on the frame and it precluded the use of the sleep system carrier below it because it is so tall. something to fiddle with at an odd moment!
if you dont plan on throwing it off a moving train the molle frame would work well for you, either with alice pack or the big molle pack(which is quite expensive and the reason i went to scheming on all this hybrid stuff).