I would think the tank (the 55g) is cycled, from your initial information. Not sure I see why others wouldn't think so, unless I'm missing something... So on to your other questions. First, considering solely the 55g.
The light will probably work fine. You do need more plants, those suitable to low or moderate light [we have fish and plant profiles, second tab fro the left in the blue bar across the top, you can browse the plants and any with low to moderate light requirements are possibles]. A few more sword plants (you have 1 right of centre), these should do well and fill in the space nicely. Move the chunk of wood in the left rear corner out, maybe left of centre sort of where the brown and black pebble rocks are? A sword plant behind and one to the left and behind of the wood would I think set it off. If you can find it, a Pygmy chain sword
(one or maybe two plants); once established they will send out runners and fill the bottom with nice little plants. And definitely floating plants. Some good ones that do well (or should) are Water Sprite
, or some of the stem plants like Brazilian Pennywort
. These are easy so I would look for them.
Nice as they appear, the three bubblers are going to hamper plant growth by driving CO2 (carbon dioxide) out of the water too fast. I suggest you remove them. The filter, does it have an adjustable outflow? If yes, this can be directed toward the left end wall so it will lessen the current down the tank but still be sufficient. And as for the inserts, you don't need carbon and other stuff with live plants so just leave them; rinse them regularly, probably weekly, to keep the water flow. You won't have cloudy water issues now. On water changes, these should be every week without fail; change 1/3 to 1/2 the tank volume. Until you get more plants, you can vacuum the gravel a bit, just lightly with the water changer. Once you have more plants, it can basically be left alone.
Do you have any plant fertilizer? A liquid fert added weekly a day after the water change should be sufficient, esp when you have more plants. I recommend Flourish Comprehensive Supplement as one of the best; you use very little so the initial cost is not really expensive long-term compared to other brands that are not as good and have to be used much more. This is the one: http://www.seachem.com/Products/prod.../Flourish.html
Salt--forget it. You do not want salt in freshwater aquaria. As for the mollies, they can manage in freshwater or brackish water and actually even in full salt water. But they will do fine in fresh, and other fish with them don't appreciate salt so leave it out.
Quick comment on the 10g, that is quite nice. I would add some floating plants though, same as above. And no salt. Salt is also harmful to plants, by the way.
Hope that helps a bit.