A few general comments.

First off, while common knowledge is that "hardness is bad", there has
been evidence of supposedly acidic loving plants flourishing in hard water
in nature and in tanks. Do a Google search on this newsgroup's archives
and I'm sure you'll find many many articles on the matter. That being
said, I think that focusing on hardness was really the wrong thing to
focus on. Of all your water problems, it is the nitrates that are of the
biggest concern. Plus mollies traditionally like hard water, so even if
they aren't showing effects now, it might appear in the future. Also, most
of the plants on your list would be perfectly fine in your type of water
as I have most of them in my 20 dGH tanks.

Now, onto the nitrates. Many of those denitrafying systems depend on
setting up colonies of anaerobic bacteria which will fix the nitrate out
of the water. Of course, the downside is you have a potential biohazard
sitting in that loop should something go wrong if those anaerobic bacteria
are of the sulfur sort. Hydrogen sulfide coming from the tank is not only
unpleasant, it can also be potentially life threatening. This leads me to
find alternative ways to fix nitrate out of the water in my own tap
situation (only 20ppm from the tap for me).

Now, on to the solutions I pondered for my own nitrate situation. First, I
stopped drinking the tap water. Nitrates are as bad for humans as they are
for fish. Then I considered an RO unit for both drinking water and
cutting the tap water on water changes, but being a poor student I really
didn't want to go that route. Bottled drinking water is plentiful in this
area, so I went with that for me, but it's a tad expensive for the fish.

For my tanks, I considered two approaches to removing the nitrates. One
was a vegetative filter. In this concept, you run the tank water through a
system with terrestrial, aquatic or bog plants planted in it. Of course,
you provide plenty of light overhead for them to grow rapidly. Many house
plants adapt well to growing in a hydroponic situation (roots in water)
and there are several which were said to be good at fixing nitrates, like
pothos/creeping charlie. Unfortunately, those house plants also happen to
be toxic to cats and I have cats who like to nibble plants, so I couldn't
go that route. Another plant said to be good was water lettuce, but I
can't find that locally (I believe it's listed as a "noxious plant" here
and therefore illegal to sell).

The second option was to heavily plant the tank with aquatic plants. I
went this route with also a heavy tolerance towards algae. Most of my new
tanks are algae wastelands to begin with but you know what? That green
string algae is great at taking up nitrates. Just a pain to pull out every
week, but it did a good job. If I could figure out how to keep it
contained, I'd set up a vegatative filter with just it. After a while, the
tank seems to reach a sort of equilibrium where the plants are better at
sopping up the new nitrates than the algae, then my algae problems
decrease to nearly nil. My similis tank took the longest to reach this
point, but that was mostly due to the similis digging up every plant
except the amazon sword, apogogeton bulbs and a few crypts that escaped
their destructive rearranging.