I have a 2.8 Pilot Parallel Pen. It simply flows too much ink when writing. I have the 6mm too and it is just fine. Is there a way to reduce ink amount?
With most fountain pens, adjusting the rate of inkflow usually comes down to adjusting the fit of the feed (the plastic piece that lies against the nib) to the nib. The bigger the airgap between the two, the faster the inkflow will be, and the smaller the airgap, the slower the inkflow. However, while it's typically very easy to adjust for more inkflow, adjusting for less can be harder and may essentially destroy the pen if you don't know what you're doing, since you may have to heat up the plastic of the feed to reform it.
Most vintage fountain pens have the nib and feed friction fit into the section of the pen (the ring that screws into the barrel), so you could try adjusting the nib up/down, relative to the feed to try and get more or less of the feed in contact with the nib, and see if that does anything. But some modern pens may have the feed/nib secured (glued) to the section in some way, so you might risk breaking it.
Given that there are also two plates in a Pilot Parallel nib, you could also consider adjusting the distance between the plates to affect inkflow. I would suggest wearing gloves, or filling the pen and testing with water, if you want to avoid staining your fingers with ink. :)