Gertrude's are quite easy to breed, especially going the route you are. I would load that tank up with plants though. Consider quadrupling the amount of moss in the tank, at the very least. It doesn't have to be attached to anything. Just toss it in & let it sink. Have you seen the 5.5g that I've got a bunch of Fundulopanchax cinnamonuem in? It's posted in my rack thread. The thing is so full of plants, seriously, it has so much guppy grass, java and peacock moss, some Anubias, and Needle Leaf Java Fern. You cant see from side to side in any direction. The other option would be to use spawning mops. I'd probably go for a couple sinking and at least on or two floaters.
As far as food, I would 100% go with baby brine shrimp. My Gertrude's go crazy for baby brine shrimp! I hatch 0.5 teaspoon per day in a pint jar. After I empty the jar, I rinse and repeat, just one jar per day. This allows me to hatch enough to feed pretty much all of my fish as much baby brine shrimp as they can eat in 10-15 min. I feed in small amounts, multiple times over the course of 10-15 minutes. I don't raise the brine shrimp past about 4-6 hours post hatching, much longer than that and they lose their nutritional value. I do have a grindal worm colony but I've been working on increasing its size over the last couple weeks. They will only be fed to the Killifish, Gertrude's, and Okefenokee Pygmy Sunfish. Daphina works well if you've got a large colony and can pick out some of the smaller sized ones. Besides the live food, I feed a second time per day using a variety of prepared foods such as the Bug Bites, and New Life Spectrum flakes. All of my fish love the Bug Bites. I have to smash them into smaller pieces from almost all of my fish, but it is easily done between my fingers.
As far as temp, I keep mine on the high side of things. Maybe 80*F(?), I'd have to toss a thermometer back in that tank to be sure. My Gertrude's are in pretty much black water, lots of leaf litter. I use RO water, when I do a water change I don't remineralize it or anything.
After removing the first set of adults, give it a few weeks. Hopefully you've let the tank build up some life in it, infusoria and such, which will allow any fry to feed before you notice them. I've got some of the NLS fry powder, seems to work alright. I feed it to my Grindal worms too. When you're putting adults in to breed for a few weeks, remember that the more fish you put in the tank the more mouths you'll have eating eggs. I know a lot of the Killifish people prefer pairs to trios for that exact reason. I think if you've got enough plant mass packed into the tank you'll do fine, even with a little bit larger group.
I added about as much as I could without severely restricting swimming space. There is a good 1" thick carpet of moss on all sides of the driftwood and covering it now. I could maybe just throw more in, but I would need to remove the hornwort I think. It's packed down pretty tight right now. They all look really happy today and they are swimming all over the place with the male going from 1 female to the next. Didn't see any real egg laying.
Are you thinking I should maybe just keep this group in here for a few weeks? My concern in getting another group in here at some point is there is no way I'm going to catch fish out of the 75 once they are in there without tearing it to pieces. I'm thinking about setting up a small 5 or 10 gallon with ANOTHER sponge filter to move moss into to hatch? Then replace the moss?? Maybe keep this group in here until I get a good group going? The biggest problem would be setting up another tank somewhere near the others and finding another outlet for air pump, heater, and light.
I don't think without a heater that they would do well. My wife freezes me out at night and has the house at 68*. I clearly need another setup to transfer egg laden moss into for hatching if I want to get any appreciable group size.
This tank has been setup for a pretty good period of time. At least 8 months. The sponge is only in there as a food source.
Maybe I could do another 11.4 and set them next to each other and use 1 longer light? Move the sponge into the tank that the moss will go into? I can figure this out. I'm super happy with these guys. They are amazingly active and healthy...
There are just too many beautiful fish.