I am a fan of AD groups or FBA roles inside of SharePoint groups regardless of the number of site collections or authentication method. It is just cleaner and cuts down the amount of maintenance for the site going forward.
Once you have an FBA MembershipProvider working (whether AspNetSqlMembershipProvider or custom), the hard part is done. Adding role support is easy by contrast.
There are many examples of getting roles to work: here, here, here, here, and here.
The point I would emphasize is in addition to registering your RoleProvider in the web.config under configuration/system.web/roleManager/providers, be sure to also add it to configuration/SharePoint/PeoplePickerWildcards:
<PeoplePickerWildcards>
<clear />
<add key="MyMembershipProvider" value="%" />
<add key="MyRoleProvider" value="%" />
</PeoplePickerWildcards>
The entries in this section are what SharePoint will use to verify the FBA role when you add it into a SharePoint group.