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An interesting feature that I found on A. sp. "Nadelstreifen", which TomC and I collected in the Leticia area in 2008, was the males' threat display pattern. Half of the vertical bars disappear.
The only true species of Apistogrmma (Apistogrammoides pucapalpaensis technically is another genus) that would work for someone with little to no experience with apistos are the truly dwarf species like A. wapisana and A. angayuara. Neither are available at the moment to my knowledge.
I guess the answer is yes and no. Think sneaker male. At the size you list - especially the smaller specimens - they can look like either sex. I personally wait until I see a dominant female who doesn't change and the same for males.
Also note that rummy-nose tetras have a reputation for being fry predators, like many blackwater tetra species. They are not a good addition to a breeding tank. In a community tank where apisto breeding is not possible (only 1 sex) they are OK.
Nitrogen source is probably plant material. I see a 'fuzzy' pod on the right of the tank. It is deteriorating and fungus is working on it. Not a real problem IMHO. Time and nitrifying bacteria is all that is needed.
When this happens to my lift tubes I use a length of 14 gauge electrical wire to unclog it (my air valves, too). I also keep extra pieces of rigid tubing in a tub of water that soften the clogs.
Ivanacara and Nannacara species are 'secretive spawners'. This means that the eggs are laid in a hidden spot, but not hidden under a covering like apistos do. The pair are probably checking out a spawning site, but it really depends on how the tank is decorated to say for sure.
Not only that but breeding sites a generally very different. Cichlasomines are mostly open substrate spawners while geophagines are mostly secretive spawners or mouthbrooders.
As Darrel has written many times pH measurements are not all that reliable in soft water. This is especially true when using most chemical and pen pH testers that aquarists use.
A. gibbiceps, being an agassizii-complex species, will show a metallic blue spot on the upper lip. A. uaupesi, being an iniridae-group species, doesn't show the lip spot.