You're a game organiser who wants to have a variety of rules depending on the type of games you want to organise. Medic, hit and respawn rules are some of the more tenuous rules which a lot of players want to clarify and also sometimes ignore just to stay in the game. But it is best to have several rules to choose from that can fit  the type of game (e.g. Milsim, regular skirmish day), or type of players (e.g. newbies, hardcore).

Hit and respawn rules are easier to understand and implement as they can be used from small to big airsoft games. Hit rules are always standard, with some minor tweakings by organisers to define "wounded" or "hit/killed." Respawn rules usually denote locations where player who are hit can proceed to and stay there for a certain duration before they can get back into the game.  Respawn can limited, time-bounded, or unlimited until an objective has been accomplished or the game is over.

Now medic rules are more loose, as organisers can play around it to fit scenarios or to make airsoft games even more intense especially milsim games. There are simple and complex medic rules so you will have to consider the game and player types. Even the medic role is not easy that in certain games I've been to, only the most fit player in a team can play the medic role fully as he needs to run one from one place to another patching his teammates up under fire. I myself avoid the role of a medic.

One suggestion for organisers: make the role of medic desirable so it's easy to find volunteer medics rather than appoint some. As how to make it desirable, I am in no position to give suggestions.

Here are two medic rules that you can implement. One is a more complex as it took a 6-minute 44-second video to explain, and one is simpler and explained in two minutes that it can be readily understood provided that airsoft players have their IFAK kits on them. Which one will I recommend, it depends. But the simpler the rule, the better and clearer for airsoft players to follow, and for a more complex one, expect instances that the rule will be challenged or even broken.

Enjoy your airsoft weekend!