1- No. You can use the radius function on the
RSA Server itself. However, the radius
function in the RSA Server is very limited,
unless, you use the RSA server appliance with
has Juniper/Steelbelt radius, then it will have
everything you need. The alternative is
to use Microsoft IAS with RSA server.
2- ACS provides much more than what you
required.
3- Firewall communicates with ACS or it can
communicate with RSA if Radius is running
on the RSA Server.