In my opinion corys are a non starter in that tank. Sorry! I am surprised no one else has mentioned that bronze corys are too big for a 15 gallon. They grow to about 8 cm and to have six of them in there alone would cause problems. If you think about it lined up head to tail 6 bronze corys would almost be the whole length of your tank! Assuming that your tank is 60x30x30cm
The only corys you could fit it there happily would be some pygmy corys. And at a push some pandas.
I know how frustrating it is when stocking a 15 gallon. Most people recommend that size as a good beginner tank size, I find it to be a betweenie size in that it is too big for just a few fish (it would look to sparse and empty) but as the same time its not big enough!
My first ever tank was 15 gallon. I started off with platys and guppys and sure enough they died one by one over the period of a few months, no matter how hard I tried to keep water quality good.
When I first stocked this tank with those fish one of my LFS told me the only fish I could have in that tank were platys guppys and mollys. When i asked about other fish they were very dismissive and said that even though there are some smaller fish, they are egg layers which you need a bigger tank for!
When all my fish died they said it was my fault even though they were the ones who said 25 fish were fine! I was stupid and nieve enough to trust them before doing my own research. Since then that shop has gone and I now go to a different LFS.
When I discovered that I could have 'nano' fish in my tank it was like a dream come true. The idea that I could have more fish for less bioload was brilliant. Of course now I know its not that simple but all turned out good in the end.
I suggest looking through nano species lists and getting some of them instead. Your tank will look much better, less maintenance and you won't regret it I promise.
Feel free to ask me any more questions
hope this has helped x