The FIRST Victor 884s specify that they come with a 12v fan and operate from 6-15v
The IFI Victor 883s and 885s specify that the 12v fan operates from 6v-16v and the 24v fan operates from 16-30v.
This site shows what fets the different IFI and Innovation FIRST speed controllers use.
http://www.enigmaindustries.com/links.htm Note that they all use different fets, with different maximum currents, however, all of them are rated for 30v.
I believe the 24v fan is bigger then the 12v fan, which makes sense as the fets will get hotter with increased voltage. So, you probably shouldn't just use the 12v fan.