I am using all three sizes of Fountain Pen Revolution’s flex nibs in three pens available on their website.
The green pen is a Guider Ebonite with a size 5 nib. The red pen is a Triveni with a 5.5 sized nib.
The brown pen is a Guider Large Ebonite with a size 6 nib.
The nice thing about the Guider pens is they have an ENORMOUS feed channel like you would find on a vintage pen and they keep up with the flex nibs with no problem.
I did have to modify the cap of the brown pen to accommodate the number 6 nib, just a heads up if you want to try it out.