There are very few who are capable of safely adding flex. Mottishaw for sure; I've had him do several of mine over the years. Mike Masuyama of Mike It Work does, IIRC. Rick Horne of Southern Scribe might be another. By and large, tho...few and far between. The big risk is, the tines have to be thinned. The nib is thus fairly susceptible to being sprung...permanently deformed, to actually just shy of cracking. They are FRAGILE, unless the nib itself was tailored for flex to begin with. Namiki Falcon's a good example.
That said, as long as there's some decent responsiveness, your best bet is a fine or XF nib. :) Sure, an XF can be scratchy because there's not much tipping, but hey, flex is best used with thoughtful writing, not dashing off quick notes.
I need another pen like I need a hole in the head...but I put an order in for one of these. Worst case, I figure...if it's got some responsiveness, but it's too scratchy...I might send it to Mottishaw and have him recut and retip this as an italic. Responsive, fine italic is a LOT of fun to write with.