I only spot-clean when siphoning during water changes as the snails and the pair of Callichthys I have pretty much removes the need. As for substrate usually I go for fine sand from the local beach. I just wash it thoroughly with warm water in a bucket and empty the bucket as many times as needed, i.e. until the water is clear. But really if you do have a local beach I recommend you get it from there. Natural sand actually helps as it contains tons of nutrients and minerals, just make sure it's collected from a healthy looking place, and do wash it first.
As for lighting, it's entirely up to the amount and types of plants you have. There are plenty of plants that will tolerate a low light setup, with for example LED lighting. The aforementioned plants that attach to driftwood for example, most kinds of moss, and crypts will thrive, if grow somewhat slower in low light. These plants and others will be great if you also have a lot of floating plants, which will also tolerate low light conditions because they're at the surface, where the light is. However if you want a truly densely planted tank you will have to upgrade to high output tube-lighting, or similar. These can be very expensive to buy new, but you can get some incredibly good deals on second-hand aquarium stuff so look into that if on a budget.
It should be mentioned that most of the plants considered low-light plants will also thrive even more in high light conditions, many species of crypt for example will grow a deep reddish/purple in high light conditions. Just keep in mind that slow growers like Anubias are prone to accumulating algae in high light conditions, especially the persistent BBA, or black brush algae, so make sure you have a proper clean-up crew. In fact, in high-light setups algae control is always a must. Some fish of the species Crossocheilus are easily the best kind of algae eating fish I know of, they will eat every species of algae it seems and are entertaining to watch. The Siamese Algae Eater is the most commonly employed, but a much less known bigger variety is Crossocheilus Reticulatus, or Reticulated Flying Fox, it grows to some 17cm average and is just as good if maybe better than the SAE at controlling algae. The Ancistrus and other Plecos are also great at cleaning plants, especially broad leaved plants. A lot of people on this forum will tell you that keeping a common or Sailfin Pleco with Polypterids is madness because of the Pleco's apparent habit of sucking on Ganoid scaled fishes' slime coat, and that only species of Ancistrus will be safe. This is nonsense, or at least partly. A great many algae eating fish, INCLUDING Bristlenose Plecos and SAE will suck on other fishes' slime coats, not only ganoid scaled fish, but all kinds of fish. The slime coat of Bichirs and Gars are just more complex, and richer in protein, and that is where the problem lies. Alot of Pleco species don't get enough proteins in the aquarium, as most think the algae and the odd algae wafer is sufficient, and that's when the slime coat problems arise. The various common Pleco species are more omnivorous and require a good deal of protein in their diet which is why they often are the first to suck on the fishes slimecoats. But Ancistrus species will also, although they're more herbivorous so will often get their protein from leftovers from other fish if not fed protein directly. With SAE it's seldom a problem as they will readily take fish flakes. My Sailfin Pleco goes crazy for market prawn, and I've never had a problem with him. The Apple snails mentioned are also great for algae control, they get big with tough shells, so they will be able to withstand playful/hungry Bichirs. I have two myself and they are entertaining to watch as they hover up to the surface for air and down again. Just be weary that a couple of Apple snail species will happily eat its way through a plated tank, the most common one being a perpetrator. Most are plant safe though, just do some research first.
As for mid-level swimmers, there are plenty to choose from. Emperor Tetras and other variants are great for a growout tank, while a few species like the Congo Tetra get big enough to stay with small Bichir species for life. The aforementioned algae eater species are really active and occupy primarily the mid level. They'll switch between chilling in the current, dutifully ridding the tank of alge, and playfully chasing each other in a manner similar to silver sharks to whom they are closely related. Some of the bigger Gourami species can also really compliment a Poly tank with their beautiful patterns and curious behaviour. The Snakeskin Gourami in particular. Then there are various Barbs, Catfish, etc. If you have the space i the future you should also get a Florida Gar, truly fascinating fish that will occupy the mid to top of the tank, and mine have been nothing but peaceful so far.
As for tank size, Ropefish tend to grow slow, but if you're getting Bichirs any time soon and upgrade shouldn't be too far away in the future, they grow fairly fast, in proper conditions you can expect an inch of growth a month, but with you current timeframe it should work out fine.
