The J&C film listed as 620 is "resized" -- 120 film that has had the spool flanges trimmed to the diameter of 620, and thinned to bring the roll down to 620 spool length as well. It will work for supply in nearly all 620 cameras, but for takeup only in a few (because the drive key in most 620 cameras is too small to engage a 120 spool).
However, htmlguru was (strangely) talking about using 620 film in a 120 camera. Generally, 620 won't fit in the supply of a 120 camera if it uses pivots in the spool center holes (the pivots won't enter the much smaller spindle holes of 620), but will if it just has a "well" you drop the roll into; 620 works for takeup in 120 cameras only if they were specifically designed to use both, of which only half a dozen models exist AFAIK.
BTW, hmtlguru, whatever you do, hold onto the 620 spools -- they're worth anything up to $10 each on eBay; you could finance your whole expriment reselling the spools!