Are there any licensing restrictions for what user licenses these can be applied to? for example sharing rules can't be applied to Customer Community licensed users.
How does read/write access work? Is it controlled by OWD or CRUD? For example if I have a user that has create, read and edit for their profile and no sharing so they can only see the records they create. If I introduce a restriction rule to include other records will they be able to read and edit or just read those additional records?
1. re: licensing - you can currently use a workaround by creating rules based on the profileid, userroleid, or a custom field (of type that is supported).
2. just as a list view or report doesn't adjust the underlying access (read vs write), scoping rules also does not adjust the record's read vs read/write access. If you don't have access to the record through OWD/CRUD, then you won't have access (since restriction rules doesn't "grant" access). Restriction Rules "scope" what the user can see based on the access setup in the organization through OWD, Sharing, Teams, Manual Sharing, Territory Management (and so forth).
