Corydoras and discus aren't the best tank mates due to the high temperatures required for the discus (28-32C). Cories like it much cooler, as do most barbs.
Dianema and Hoplosternum catfish grow to 4-6 inches and are fine in warm water. Both have small mouths and are very peaceful. Males have a longer, thicker pectoral fin ray compared to females. It is usually a different colour too (orange or brown).
Black Lancer catfish (
Bagrichthys macracanthus) and bumblebee catfish (can't find the scientific name) grow to 6-8 inches and are also fine in warm water. The black lancer catfish has a smaller mouth, whereas the bumblebee catfish might knock off the tetras at night.
Male black lancer catfish have a penis just in front of the anal fin.
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Instead of getting bristlenose catfish, why not look at some of the more unusual suckermouth catfish like Whiptails (Loricaria sp), Twig catfish (Farlowella sp), Peckoltia sp, or even some of the different coloured bristlenose (zebra ancistrus or peppermint plecos). They are much more interesting and would probably breed in the tank (due to the tank size). They are a bit harder to find but most shops can get them, they just don't. Any young you produce can easily be sold.
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Big rainbowfish will eat small narrow tetras so avoid mixing them together. Smaller rainbowfish will be fine with tetras and warm water.
Ruby, rosy and Odessa barbs will be fine with rainbowfish.
The following link has information about rainbowfish.
http://rainbowfish.angfaqld.org.au/Melano.htm
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If you are going to keep discus, either get 1, or get 6-10. If you have 4 or 5, then 2 will pair up and bully the others. It will be less of an issue in your tank due to the tank size, but it will still happen. Having more fish will spread the aggression out over all the fish.
Discus generally prefer soft acid water (GH below 150ppm and a pH below 7.0), whereas most rainbowfish prefer harder alkaline water (GH above 150 and a pH above 7.0).
Iriatherina werneri and Rhadinocentrus ornatus will be fine in soft acid water and some of the other bigger rainbowfish will be too, but Melanotaenia boesemani, Glossolepis incisus and a few others come from hard alkaline water and won't do well in soft acid water.