I have heard that, on some of the pre-Hagar (recorded) stuff, Eddie would use both Nylon and steel picks off and on. The reason for the steel picks was that they have a tendency to be brighter and really pull out those ghost harmonics that Eddie was so famous for in the early days. As for when he played live, I would guess he stuck to nylon. Steel picks are hard on strings and Eddie has a tendency to get mean with those suckers which would mean more broken strings during a live show if he was to use steel.
However, I'm not ceratin as to what he uses, during recording, today. I saw them on their last tour and he only used nylon, when playing live.
I wouldn't bank on using what Eddie uses for your own playing, however. Just find something you are comfortable with. If you are doing a lot of strumming, use a lighter guage pick...something with some flexibility. For faster picking and to ring out those pinch harmonics
, you'll want picks that are fairly stiff.