Hi,
If your switch is 1Gbit, chances are very good that it also support 100Mbit and 10Mbit connections.
If your fileserver has a Gbit NIC, you should theoretically be able to service multiple clients at 100Mbit. The switch will only "downgrade" the clients connected at 100Mbit, not the whole switch.
Having said that - your fileserver must be fairly powerful to serve up a sustained feed of 1Gbit/sec to multiple clients, especially if your clients are accessing different sets of data which will cause your disks to seek more than just read.
Also, most NAS systems out there simply can't serve up more than about 120Mbit/sec. See
http://www.smallnetbuilder.com/content/view/26378/75/1/10/ for more info on several NAS boxes' throughput.
--deckert