First, I believe you are referencing LSPDFR, standing for Los Santos Police Department First Response, instead of LCPDFR meaning Libery City instead of Los Santos. LCPDFR is for GTA IV while GTA V uses LSPDFR.
Second, most of the content LSPDFR offers is about preventing drugs, violence, etc. So there wouldn't be a way to "turn off" the violence, but if you're OK with exposing him to the criminal world in a positive light, i.e stopping crime instead of committing it, then I'd say LSPDFR is for you!
Third, most mods are compatible with each other.
Fourth, YES, installing mods WILL block access to online modes and YOU WILL BE BANNED if you use modifications in online play. HOWEVER, if you make a copy of the game directory, you can switch back and forth as you please. Below I have included a tutorial by one of my favorite LSPDFR YouTubers that explains exactly how to install LSPDFR. If you like, contact me on discord if you need any further help.
Happy playing!
-Juice1800
Discord: Juice#0451