The bottom line -- you need to use the same ink. I like my fine (and I still do), but the medium I had borrowed had a much "wetter" ink and spread out a lot when we first gave the two a try with copier paper. The ink in his medium was Sailor Manyo Yu. We found some Rodia paper, which is almost polished, and even then the wet ink spread quite a lot.
I'll stick with my fine nib and Diamine Oxblood ink!
My son is suggesting Maruman Mnemosyne paper. He likes an A5 notebook (and I do also). You can get a "N195A Notebook - A5 - 7 mm Lined". He likes this more than Rodia, which is a very polished glass-like paper.
Tom's Pen Info / tom@mmto.org