Are you saying that the big gravel would be okay so long as I mix in some
Fluorite (and/or Onyx) and peat to it, and then top it off with pure Fluorite
or are you suggesting that I remove the big gravel altogether and replace it
with the fluorite/peat mix on the bottom with Fluorite only on top instead?

I don't like mixed gravels one bit. So I would use the flourite or
onyx sand 100% in this case. It won't cost too much more to doit and
then you never wonder about the gravel ever again.

Also, are you recommending that I use both Fluorite and Onyx, or one or the
other?

I'd use just one or the other, no mixing.
Imagine trying to separate them in the future should you not like the
look:-)

Other than adding CO2, should I worry about adding any additional supplements?
I have plenty of Seachem Fluorish and root tabs...

The above recommendations will supply all you need for the plant
roots.
I use KNO3, K2SO4, KH2PO4(For the N-P-K fertizer no# you see on the
sides of bags) and Traces dosed directly to the water.
Regular fertilzer has high NH4 which is real bad stuff as it causes
algae blooms if even a trace hangs out in the tank too long.

You can get the KNO3 at a number of local sources and likely the
K2SO4(Sulfur of potash) and for PO4 the Fleet enima's sold in drug
stores are a good source of PO4(sodium phosphate). You can use TMG,
Flourish etc for the traces.

Your light level is moderately low so once twice a week at most dosing
is all you'd have to do.
After a weekly 40-50% water change add:

2-3 mls of Flourish,
1/4 teaspoon of K2SO4
1/4 teaspoon of KNO3
1-2 drops of PO4 if the tank is doing good.
That is about it. You may need to add this once more during the week,
like 4 days after the water change.

I would tremendously appreciate if you would clarify these minor points for
me...When I am ready to do this I intend to follow your advice carefully.
Erica
http://members.aol.com/_ht_a/mitoem/mitoem/index.htm

There is a lot to be said for good old fashion getting in there and
removing any algae etc and pruning the plants and using a net to
remove any junk floating around. Adding herbivores are good, but I
don't rely on them to take care of the algae, just added insurance.

The rest is keeping the CO2 in a good range, adding nutrients
regularly and doing weekly water changes.

It's not complex and anyone can do it. It becomes old hat after a few
weeks. Being lazy and not pruning often gets everyone at least once
over the years, but a simple repeat and back to the old routine is all
it takes to get things back up to snuff.

Regards,
Tom Barr