1) Lighting - I currently have a 192 watt system that has 1-96w 10k bulb, 1-96w actinic. I will be replacing the actinic with another 96w bulb. Should I make this one a 6700 bulb or does it matter whether it is 6700 or 10k? Should I change the 10k bulb to a 6700?
6700K is really ideal. The actinic should definitely be replaced with a 6700K bulb. The 10,000K could also be replaced, but it's not completely useless for a planted tank so it kind of depends on your budget.
2) Substrate - I will be using 5 bags of EcoComplete for the substrate. I believe that will give me approximately 2 inches depth in my tank. I want to ultimately have 4 or 5 inches in the back sloping to 2 inches in front. What should I put under the EcoComplete in the back to bring it up to 4-5 inches? Anything reasonably inexpensive? Laterite maybe?
If you don't want to spend all that money getting 4-5 inches of EcoComplete in the back, you could always mix together EcoComplete and a dark colored natural gravel. That'd be a bit cheaper.
3) Current- Current is really important in the saltwater world. What about Freshwater?
I get the feeling, maybe erroneously, that current isn't as important in freshwater, though it is important to a degree for Oxygen exchange. I do want the plants to have a little swaying motion though so I was considering putting in a low output powerhead for circulation. Any pros/cons to this setup?
Circulation isn't as important in freshwater as it is in saltwater. If you've got leftover powerheads you may as well use them but they aren't essential. Also - what sort of filter will you be using? If you're using a canister filter (which is usually recommended for planted tanks as it doesn't disturb the surface) you can place the input/output so as to create a decent amount of current in the tank.
4) Plants - I am considering having low growth plants covering the substrate either entirely or virtually so. Is there any reason this is NOT a good idea? Any reason this is a good idea?
Sounds like a plan. Most of the better looking heavily planted tanks have some sort of short, grass-like plant out in front in the open swimming area, with taller plants around the sides and in back.
5) Water - In the saltwater world, RO/DI water is recommended. This doesn't seem to be the case in the Freshwater world. Is this true? Is tap water better than RO/DI? Perhaps because too much has been removed from tap water?
Probably no matter what is used, water needs to have some conditioner put into it either to put something back into the water that was removed (RO/DI) or to take something out (chlorine, etc) that shouldn't be in the water (Tap water).
In a freshwater tank, you really have no way of removing nitrates from the tank other than by plant metabolism and water changes. Unless your tank is very heavily stocked with plants and has very few fish, you'll need to do regular water changes to keep the nitrates at bay (of course at least monthly water changes are recommended anyway to replace nutrients). I'd say you're putting a lot more water into your tank with freshwater than saltwater as water changes are more frequent. For this reason, it's better to keep costs low. It's much cheaper to use a water conditioner on your tap water than purchase RO/DI and use a supplement to replace lost trace elements. Most freshwater fish are adaptable to a wide variety of water chemistries, so tap water is fine in most instances.
6) Cleaning saltwater residue - What is the best method of cleaning out the current aquarium to make sure the no salt residue remains?
A good ol' razorblade is your best friend here. Vinegar can also help clean off some of the calcium buildup.
7) Airstones - Are they necessary?
Not really. As was pointed out, in a planted tank that's being CO2-injected, disrupting the surface leads to CO2 loss so airstones can be detrimental. If you dose something like Flourish Excel for your plants' carbon needs, disrupting the surface isn't as big of a deal and airstones are just fine. They're mainly aesthetic in function.
Those are some of the major questions. Here are a couple of miscellaneous question:
1) Are gloves encourage when working in a freshwater tank? They are in saltwater tanks.
Nope, not unless you've got a shellfish allergy and you're keeping shellfish in the tank, or something like that. Or if you've got a big nasty cichlid who likes nibbling on you during tank maintenance.
2) My wife liked the though of Bamboo in the tank (roots only in the water). What are the pros/cons of this.
Again...bamboo is not an aquatic plant. You'd need very tall bamboo and you'd need to not have a cover on the tank (or a lowered water level) in order for this to work. There are many fully aquatic plants that will work very well, though.
3) Frogs - We saw frogs in a store that apparently live underwater all the time. Are there any negatives in getting one frog in the tank?
55g is rather tall for frogs as they need to surface to breathe. They also will need to be spot-fed with something like a turkey baster or else they'll slowly starve as the fish steal all of their food. If you want frogs, get African dwarf frogs and not African clawed frogs. The clawed frogs will have distinct black claws on the toes of their hind legs. These guys reach baseball size and will eat any fish that fit in their large mouths. The dwarf frogs stay smaller and are fish safe.
4) Black versus blue background - Any thoughts?
Black really works best for showing off your plants and fish, in my opinion.
Hope that helped!