Maisy, Just a heads up. Make sure that you are into the consideration of the species of Corydoras you are talking about. Yes dwarf cories are schooling fish, but they ideally should be in groups of 6+ to be active and seen in the open and swimming, dwarf Corydoras are shy timid fish that really should have groups of 6+. While other larger species that aren't as timid and shy due to size can be happily housed in groups of 4+ so once again, just take into consideration of the species of fish concerned in the situation when there can be different variables
To the real question. In a 15 gallon I dont think that you could hold 3 proper schools of each fish, I would recommend something along the lines of
8 dwarf Corydoras
1 male Betta
Man I wouldn't put more than a betta and one school in my 15 gallon. That's what I was suggested. :/
Scratch the dwarf cories. Instead of harelquin rasbora, try to find espei rasbora, they look the same but are much tinier and would have less bioload. Cories aren't that colourful anyways ;)
Or scratch the rasbora, and have the 7 dwarf cories in, as bottom dwellers that will balance the tank more.
Note this is VERY heavily stocked. I'd buy 4 dwarf cories, wait about a week, maybe longer, and buy 3 more dwarfies, just to not totally ruin your cycle. Is your tank cycled? All cories are VERY sensitive to dirty water and you will most likely have very little success with them in an uncycled tank.
I've got fast growing plants. The plants are the main reason for the darn tank. My 1 betta fish was perfectly happy with his 1 gallon tank. The plants got too happy. The tank has been cycled. I bought a bacteria culture and used a scoop of cultured water from the one gallon I have in order to give the 15 gallon a boost. I've got 2 betta bulb plants (I've heard theories from them being lily to apognates of some variety). I do have an oblong leaf floating at the waters surface like a lily pad; ludwidgia repens; and Java Fern. More plants will come in the future. I'm doing THE RASBORAS (next Sat.), as for a cleaning crew I'm thinking Ghost Shrimp again, they eat everything! I don't really care if they (the ghost shrimp) get eaten either, and this time they can't escape through my filter!!!! MY PLANTS are HUNGRY.....Oh yes, it has been observed that plants LOVE ghost shrimp. For some reason the plants grow faster after being colonized by these guys. Probably because the shrimp keep the plant clean....
The 15 gallon tank, bigger than what I was considering, only came into fruition because it was a steal... and the lady at the store was kinda cute.... BUT THE TANK was a steal. Bigger than what I really wanted and made of glass not acrylic. So it is heavier than I wanted and the quality is better than I wanted. I wanted something cheapo, in case stuff happens. It's going to be a pain in the neck to move this puppy when I relocate too.... Unfortunately it was cheaper than the small stuff and came with a really cool heater and other accessories that alone run a pretty penny. So I bought the sucka.