Bryant,
If I understand correcttly , you are trying to purchase a 10-to-Unlimited User license for the ASA5505 firewall and want to know if that same license can be utilized by PIX501 platform, the answer is no. This is why, when you purchase a license in this case for your ASA5505 you will be geting a Product Authorization Key (Pak) , in which you will then provide this key at http://www.cisco.com/go/license , the key is especific to platforms , that is , once you enter the (Pak) in mentioned link above you will need to provide ASA5505 serial number which is provided by the output of "show version", then Cisco will generate an Activation Key for your ASA5505 firewall and email it to you. This activation Key can be only use in the ASA5505 firewall serial number you provided.
The same principle applies if you were geting the License for the PIX501, it cannot be utilized on the ASA5505.
Regards
Jorge Rodriguez