Any standard expression will work (Moog, Dunlop, Roland).
I'm not sure I understand exactly what you're asking so I'll answer as I interpret your question. In order to get the most control you need at least an expression pedal and some means of connecting the expression pedal to the Fi via 5-pin MIDI port. If you are a Source Audio user then their Hub will work for that but you'll then need something to connect to that if you want to be able to change presets remotely.
If you want complete control over everything then you need a MIDI controller capable of sending multiple CC messages. I highly recommend the Morningstar Engineering MC6 or MC8 for maximum control. You can hook up 1-4 expression pedals (depending on the unit in question) or have one expression pedal be capable of controlling up to 16 different parameters (simultaneously if you so desire). With it you can also use MIDI CCs to control the filter cutoff, synth volume, master volume, glide rate (if using a keyboard), LFO depth to filter and VCO as standard. Furthermore, via the four flexi slots you can control up to any other four parameters in real time via MIDI CCs.
My honest advice is buy an MC6 and an expression pedal and you'll have all the control you could wish for.