I do not understand the question
the statement "ASA 5505 need a router between every different networks." is NOT true !
also with an 5505 you are able to allow between two internal networks without using an external router
- you give both network the same security level
- you need 1:1 NAT between both networks
- you need access rules between both networks
or enable "allow access between networks with same security level"
basically same for the FPR