Hi welcome to the forums!
1st: there is no set velocity for fields in general. Having said that, there is a "standard" FPS rule of thumb : usually between 350-400. 400 for outdoor games is the max I have seen because of the size of the field. 350 max for indoors because of the risk of Close Quarter Battle/Combat (CQB/C).
2nd: It is easy. Usually for AEGs, just a swap of a spring will do the trick (widely available and usually cheap). Since winter is coming the 350 fps rule might apply more. if you don't want to tinker with your gun too much, you should probably go for less FPS. But you should figure that by yourself (NOTE: if you get a cheaper ACM (All China Made) gun FPS will most likely be around the 400 mark) do not ask me how or where to get it though. Explanation on why is coming.
3rd: You will be ask to sign a waiver form before each game, stating that organizers and field owners CANNOT be held responsible for any injuries during games. Even without that waiver, here in Quebec, you couldn't sue nobody for injuries (on marche sur la loi du Gros Bon Sens). So glasses are fine, it's your eyes, mouth, ears, please be careful.
4th: I told you not to ask me about guns because the 1st person you'll have to see to buy on here is an Age Verifier that will confirm your age and will grant you an access to the classifieds and retailers on this forum. AV must be done in person.
Hope this will answer your questions!