Hi,
We will need to use shell command authorization sets with command authorization.
More about shell command sets at :-
http://www.cisco.com/univercd/cc/td/doc/product/access/acs_soft/csacs4nt/acs33/user/g.htm#wp480029
command accounting on Pix 7.x can be enabled by :-
aaa authorization command LOCAL
I have assumed here that authentication and exec authorization is already working between the PIX and ACS.
Regards,
Vivek