epacaguei will Google Play be able to tell what other apps I have installed in that profile (aka, snapchat, facebook and facebook messenger)?
Yes. It can and will check what other apps are installed.
epacaguei if I log into google play services and then log into gmail it'll recognise the user, so there must be some communication even in between apps while still being mostly sandboxed.
The account login information is saved in Settings > Passwords & accounts, so both Gmail and Google Play will use that to log in.
Also, yes, apps can communicate if they're both set up to do so. Or, in other words, communication has to be mutual.
epacaguei You can already edit and move around the icons that are made available by graphene, but I don't know if it's possible to add new shortcuts.
epacaguei quick access to turning on either proton or my own personal network vpn.
I have a ProtonVPN tile available that says "Quick connect". I have never used it. Maybe it's what you're looking for? Though keep in mind if you plan on toggling it on and off, the default setup for VPNs on GrapheneOS is to block connections that don't go through the VPN. If you want to change that you can in Settings > Network & internet > VPN > *the gear next to your VPN* > Block connections without VPN.