if i right understand u,, then yes u apply the firewall rules on the outside NIC where u get internet traffic..
about pinging , u should be able to successfuly ping anytime, b4 and after installing samba
caz its something more basic than the samba service, in more technical words , samba works
on the application layer and the ping on the transport layer,, so all the protocols (ip in this situation) should constantly be able to serve all of the layers and applications above it.. got it?
it should ping and it should response all the time!! as pinging is living