Is it going to cause the corrosion when connecting a galvanized steel nipple to a brass ball valve?

No, It's done all the time with brass shut off valves connected to galvanized pipe. Cheers, Tom

Tom is right. In the factory I once managed, there were some iron body valves, but I am sure more of a cost factor than corrosion. What would a flanged 8'' brass gate valve cost? I think some of say 1'' steam valves had iron bodies too.

And yes, use ball valves anywhere you can.