Guys, since you aren't playing at a real field with insurance and rules and such, why not sort out your own shooting distances and velocities the old fashioned way? In other words, shoot each other at certain distances with the types of guns and see which hurts too much for which range. Paintball is quite a bit different than airsoft, paintballs are heavier and carry weight better, but hit with a larger surface area and break. Airsoft BBs are smaller, go faster, don't hit with the same impact as a paintball, but can break the skin and imbed themselves if too close or too high of a velocity.
Literally, is like posting the question "How much pain can my friends and I endure?", we don't know, up to you guys to decide your own rules, just expect lots of welts, bleeders and bruises. You'll sort it out.