I think that the Classic, and by extension Mini and Eco, nib is much closer to the FPR 5.5 than to the 5. The main difference between the 580's nib and these other pens' seems to be the width of the shoulders, not the length itself.
Yep, I think you're right. A quick comparison of the FPR #5 and #5.5 to a spare (damaged) 580 nib looked like this:
There's less of a difference in length between the nibs than I remembered - but the #5 is definitely a *little* bit shorter.
The 'wingspan' of the two FPR nibs was very similar (roughly 7mm), compared with 8mm for the TWSBI Diamond 580 nib; the Mini / Classic / Eco nib is closer to 6mm. You *can* fit a 580 nib inside the cap of a Mini, though - so I can't imagine the 7mm wingspan of the FPR nibs being a problem.
Still not game to try and swap out my Diamond Mini nib - they're jammed in pretty tight - but I'm fairly confident the #5.5 (or a #, though they're not as pretty!) will be a good fit.