How much impurity you can get away with depends on the particular process Caswell is using. I don't think the aluminum would be a problem but iron causes black spots on parts if it builds up in the bath. Other impurities like copper, lead, cadmium, etc can cause problems too in *very* small quantities. Most metals are more 'noble' than zinc (though not aluminum) so they will be codeposited with it. You'd really have to try it to find out whether there'd be a problem or not.
In the end, it might just be easier to buy anodes you know are good. Standard zinc anodes for acid zinc plating are 99.99% pure. For example:
http://www.palminc.com/metals/zinc.htm