I like to leave the fxp interface in the base routing instance. And then create traffic routing-instances for all the other functions on the device. This gives the mgmt a separate route table and access as you suggest. And keeps all transit traffic with their own routing tables.
Whether our not the external is complete separation for OOB would depend on the situation and expense of the solution.
But if the network is literally a single flat vlan I guess there would be little point to the configuration. I might be inclined to create the routing separation anyway and just use the same subnet on the fxp interfaces. This would allow a migration more easily down the road.
Steve Puluka BSEET - Juniper Ambassador IP Architect - DQE Communications Pittsburgh, PA (Metro Ethernet & ISP) http://puluka.com/home