Two questions for anyone out there that may know about this...I have a Line 6 FM4 Flilter Modeler pedal and want to get an expression pedal for it. Is the Line 6 brand expression pedal (I think it's the EX-1) the only pedal that works with the FM4?

I also have a Boss RE-20 Space Echo and also want an expression pedal for that...could I get one pedal that'll work for both? It'd be sweet if I could just pull the 1/4" cable out of one pedal and into the other depending on what song we're playing...can I use any expression pedal, or does the Line 6 need the Line 6 and the Boss a Boss expression pedal?

Just get any ol' Expression pedal.
Infact if you want you can get a Volume pedal and just use that as an expression pedal
That way it could be a volume, an expression for your FM4 or an expression for your RE20.
It will also be sturdyer.