Dawn - Fry for feeding are the primary goal, but at the same time, this tank will be in my living room, so appearances are important. A large group of black mollies (or wild type latipinnas) in the midwater would make a pretty stunning display by themselves, but I would like to add some point of interest to the bottom strata as well, and, if that fish were also a good breeder, so much the better.
TFH had an article a couple of monthss ago in their livebearer's corner about setting up an all Livebearer community, so one of the ideas I have is to add limias, as I understand they stick closer to the substrate.
That could work, but you may find that with mollys in the tank, they may tend to spend a lot more time in mid to upper levels of the tank with the mollys. Also being live bearing, watch your male/female ratios... as much as you want the fry for feeders, you also don't want to be over run with so many that you have water quality problems. My 90 gallon was devoted to breeding mollys, and I started with 6 females and 2 males. Lo and behold, one of my females even turned male on me, and it only took 3 months to end up with over 200 fry in that tank, and needing to do daily water changes to keep up with waste levels. I was not using the fry as feeders, but about 50 fry/month were going to the store to trade for plants. If using them as feeders, 50 fry is a lot of fish for a month... and in a 55 gallon, you're stocking limits will be less than mine were. When you count your female/male ratios, be sure to include ALL livebearing fish in the tank and expect 30 - 50 fry every month from each female, if not more. They are each capable of having up to 100+ each month, and in retaining sperm from mating for at least 3 - 4 spawns. Also remember that any livebearing female you bring home from a store is likely to already be pregnant, and could have 3 - 4 spawns before every having a male in your tank. You might want to consider starting with females alone, and add your males later...
Another is to add cories and divide the substrate up into zones of sand and large pebbles that cory fry could get down between to grow out a bit.
I would not suggest the sand. Corys can do just fine in small to medium size gravel. Sand and gravel will bleed together and mix, sand will circulate in the water whenever disturbed, which can lead to filter problems, and sand is very difficult to clean waste out of. Many years ago I listened to someone who suggested that to me, and I learned the hard way it doesn't work. Sand also will not benefit your biological filtration, it's too dense for the bacteria to culture in normally, and sand is too dense for most plants to grow in very well. Anythng with a root structure is a no no in a sand substrate long term. I have raised crypts in sand, but it was very difficult and they didn't grow as well as in gravels.
Yet another is to discard the idea of a second breeding species entirely, make the tank lightly brackish (<1.005) and add a 1 or 2 violet gobies. I understand that they can get 20" long, but it is my understanding that this rarely happens, and they usually top out between 12 and 18".
I would skip the violet gobys. Size is one issue, of course. The reason most don't grow to full size in a smaller tank is the same reason any other fish won't grow... growth gets stunted due to water quality issues and you end up with internal problems and damage. Size restrictions like that in an aquarium will also cause physical deformities. Another issue is that the violet gobys will also feed on your other fish, and if it fits into the mouth, it's fair game. This is not conducive with keeping fry or anything smaller than will fit into the mouth of the goby. Overall, bad idea with this one.
For fry production purposes, cichlids are right out, I expect, as they would very likely eat the molly fry, and it seems to me that most cichlids that would breed in hard water would also be tough on plants.
Correct. Cichlids will chew up adult mollys, which means no fry to eat. As for plants, if the cichlids are African, then expect plants to be eaten. If they are Central or South American, most will need softer water and they still tend to do a large amount of digging, which can make keeping a planted tank a challenge. Dwarfs would be the only cichlids to do really well in a planted tank, and again, you're looking at pretty soft water.
Regardless of what else goes in there, I'm thinking that throwing about 20 ghost shrimp in might be a worthwhile endeavor, since they might well breed. I would start with just 10 and keep an eye out for any long claws that might have slipped in.
That sounds like a good idea. With enough plant cover, ghost shrimp tend to breed pretty easily. Expect it to take a few months before you see anything for fry, though. It takes a while for ghosties to settle into their environment, and you'll want to make sure you have at least 2-3 males out of the 10 to ensure breeding success.
Another thought: Florida blue-fin killifish. These guys come in with ghost shrimp shipments pretty frequently, and my LFS sells them as "darters" for a buck.
Now that could work... but watch the fry population... they will still feed on some of the smallest of fry unless they're very well fed... and that means a lot of water changes.
Darters... Now there's an idea. That would take some considerable research that may ultimately prove fruitless, but aren't there several species of southwestern US, northern Mexican darters that might do well in a tank with mollies? Or would the current need to be too strong?
I would rule out darters alongside of the violet gobys. Again you are looking at something with habitat requirements that will differ greatly from the mollys, and you're also looking at something that no matter how small when you get it, will eat your fry. Daters tend to have very healty appetites when there is plenty of live food around to amuse themselves with. I wouldn't mix them for the situation you're trying to create.