I wish. The water is not cheap. It's distilled..and then
some...etc...etc...it is as pure as water can be... Thus a big reason my cost
for ink is the highest, and profit most likely the lowest (competitors of
course have not showed me their books! ;-)

For rinsing out my 61's - I actually put the filler end in a surgical tube (the
stretch latex kind) and put the other end over the faucet - then run the water
through the pen at low pressure for a few minutes - until the water runs clear.
You can also do this holding the filler end with your hand sealing enough to
push the flow through - but you'll waste more water that way...and might spray
it clear out of the sink and make a mess. If you do this before long term
storage and before switching inks - the pen should be fine for a long time.

The main ink I fear concerning mixing is a Ukrainian ink that is highly acidic
and has other unsavory chemicals - it also smells slightly of rotten eggs.
Avoid that and also avoid the Chinese ink that smells like turpentine paint.
Both have salts and high acid content. I have no other way of identifying them
- as my samples arrived in medical bottles and not original labeled bottles.

Mixing - don't bother mixing the bottles... When using vintage inks with
Frank, he noted how any dust and debris that is attracted to the feed and nib
as one writes would get washed off and in a good ink, would settle to the
bottom of the bottle...leaving the top layer of ink clean (I always said "Why
not rinse before filling"...but of course most people rinse on the 3rd or 4th
filling). Notably in many vintage inks that were high dye content inks, it was
also found some concentrated dye would coat the base of a bottle's glass more
than the sides and top (enough so that when dried up you could see through the
sides of the bottle, but not see through the base). Just fill from the top -
don't mix... The very bottom of the bottle is the same as the bottom of a wine
barrel. The brim vrs. the dregs.