My attic unit already has a wire connected to C, can I still connect a wifi thermostat?
Yes.

If you have an air conditioner, the C wire is used to connect the relay in the outside condenser unit. 

Technically speaking, if the transformer in your furnace is too small (eg: JUST big enough to power the control board, relays, and the condenser relay) then it's possible that the thermostat could push it over the edge, causing your furnace to not work and/or behave strangely. However, the amount of power we're talking about here is pretty small. Unless you're connecting a thermostat with a monster color touchscreen to a 20-year-old furnace/air handler, chances are you won't have an issue.