A PIX501 should be able to connect to an ASA5520 on a S2S VPN if they are on the same version of code, etc. correct?
They both follow the RFCs, regardless of code version. You're good to go.
They do not need to be on the same version of code. The latest code for a PIX501 is 6.3(5) and an ASA can't run code that low. There is no problem swith each having different spec's.
Hope it helps.