You FOR SURE can have - FC (which makes your spells take longer than naked). Go get screamed at by the bard masters in blackthorns, which give like FC -5 as a penalty ... and then watch it take you 5 seconds to cast close wounds. This happens to me all the time. I wear 0 FC, get howled at, and then it takes a VERY noticeable longer time to cast my Chiv spells. It isn't that FC can only go negative for mage spells, for instance.
If your suit has 0 FC, then you put on a shield with FC -1, I believe that your character will take longer to cast spells than without the shield. Since it is only .25s it would be quite difficult to keep track of this with a stopwatch, but I bet that you could programmatically prove this fairly easily.
The only way my statement would not be true were if there was a difference between the FC floor for items vs affects.
Perhaps it would be noticeable if you put on a FC-1 shield and an FC -1 weapon, which would make your castings a fill .5s slower.