The Exchange Server setup program requires a writeable Global Catalog Domain Controller (GC) in the Active Directory Site where the server is. A Read-Only Domain Controller (RODC) will not be used. You can't get around this requirement. Either temporarily promote a machine in the Site to be a DC or move the new Exchange Server machine to a different site.
Once the Exchange Server machine is operational you can remove the GC from the Site though this is strongly recommended against. Almost everything Exchange Server machines do (particularly the Client Access role) requires GC lookups.
Installing an Exchange Server does not require the Schema Master FSMO to be in the same site (or even accessible). Extending the schema (forestprep) does require the Schema Master to be operational and accessible, but this is an operation you do once per Active Directory forest before you do anything else with Exchange Server.