For a hot-wire, the steppers could be much smaller than those, and would work well. Remember, the magic to a hot-wire is to make the wire hot enough to go through the foam, but not hot enough to create smoke. If there's smoke, you're WAY too hot! If the wire is at the correct heat, it will leave small strands of "angel hair" in the cut.
Once you reach the correct heat, the
CNC will move the wire at about 3 - 5 inches per minute, and there isn't a lot of pulling force required.
Also, get yourself some Rene foam cutting wire - this stuff is light-years better than Ni-chrome or safety wire or piano wire, trust me. I bought some from these guys and I'll never waste my time with anything else.
SkyKing Rene foam cutting wire