Finding fish for the upper third/surface level is not that easy, but there are some. Hatchetfish obviously. I have had all known/described species at one time or another. Those in Carnegiella are the smallest, and remain at the surface; Carnegiella strigata (Marble Hatchetfish) is probably the most common of these, but there is also C. marthae (Black Winged Hatchetfish). The smallest, C. myersi, is also very delicate and sensitive, and personally I would not put this species in large tanks. Either of the first two, in a group of 12-15, and you can combine the two species (have at least 8 of each if both). The species in Gasteropelecus and Thoracocharax are a bit larger, all are basically silver with some patterning depending upon species, and tend to remain just below the surface. In my experience these interact a bit more frequently that the fish in Carnegiella. A group of 8-12 of whichever species of these, and here too you can mix species but have several of each species if combined.
Other near-surface characins include the Penguin Tetra (there are two or three species in Thayeria, varying in pattern). The "splashing" tetras (species in the genus Copella) and the closely related Pyrrhulina species are not seen much these days. This family, the Lebiasinidae, also contains the pencilfishes in Nannostomus, and one frequently available is N. beckfordi which is known for the almost continual displays of two (sometimes three) males. One negative is that this species is very territorial and males often take a dislike to any fish in their space, which is the upper third of the tank. Hatchetfish will be nipped to death, and I have seen otos chased lower down. Among themselves though, in a group of 12-15, they are a lively and colourful option for the upper level.
Most if not all of the disk-shaped tetras are lower half level fish. Given the space here, the larger Bleeding Heart is worth considering; a group of 9-12 would make a nice display mid-level. If you did decide on angelfish or similar, this is not a species to have as they can sometimes get a bit nippy with very sedate long-fin fish. But otherwise quite beautiful.
Substrate fish...none of these will do much with cleaning up food as they have specific dietary requirements. Snails are better at clean-up, or maybe shrimp.
Bolivian Ram, Mikrogeophagus altispinosus. This is a fish that does very well as a single individual. Observations in the habitat suggest it may live in isolation except when spawning. A lone male certainly makes a fine peaceful addition to any South American tank that is at least 3 feet in length (your 90g is presumably 4 feet). My last one was into his ninth year when he died, pretty good for a fish with a normal lifespan of 4-5 years. If you have a pair, meaning male/female, they must select each other from a group and bond, or the female will not last long. Many years ago I introduced a lovely female to my 5-foot tank with the male, and in hindsight I now realize there were signs from the outset that it would not last long. They spawned four times, but the male then killed the female. They had not selected one another. A group introduced at the same time as juveniles might manage, but males are territorial and they can exhibit this very aggressively when they choose. My lone male considered the 5-foot tank his space, and all the other 125 or so fish knew and respected this.
