The 5/8" is a pretty common height for many bluegrass banjos. I think bridges can go up to about 3/4" - but I've never seen many lower than 1/2" (although the sandpaper trick can get you as low as you want!)

The guy at the shop was probably going by what he thought the average player would use. I think I've got an old bakelite Harmony 5-string lurking under my bed. I'll try to remember to see what it's got on it.

If you find out you really are serious about being a banjo player, you'll want to move on up from the bakelite banjo fairly soon. I couldn't believe how much better I got when I finally got rid of my old $60 beater and into a better built, better handling, and better sounding banjo. Good luck !