Yes, in my experience Cisco and ScreenOS will not correctly auto-neg a 100 / full connection and they must be manually set on both sides.
The screenOS command is this substituting your interface.
set interface ethernet0/5 phy full 100mb
This will bounce the physical connection and all traffic crossing it so should be done in a maintenance window. There is a small chance the link would not come back up. But generally if the Cisco is already set to fixed 100/full you should have no issue.