I have read both threads. I must say- I am glad you came to the realization that you need to make some changes. You are in Singapore, so you can probably find a few people really into betta keeping and/or expertise. They might offer medicine, knowledge or spare tanks. It costs you nothing to do the research to find these people.
So your tank - is it like this - http://www.adgshop.com/30cm_Cube_Garden_p/102-870.htm
7.5 gallons approximately. This would be fine for the 2 female bettas (though there would be fighting with less than 4 females).
Craft stores, dollar stores, markets - anyone selling buckets and tanks and bowls would benefit you at this point. Since you don't need heaters for all of them (since your house temp is already pretty warm) this will be fine to start medicating all the fish. I don't remember which other fish are in there but how many of each type? Oto's and cories and what else?
You might want to tread carefully around the whole waiting for them to die thing. Most members here might disagree that it is a good idea since you are stepping around the fact that you have an over population problem now which has caused some serious disease. Most people here are for the medication path: medicate and treat the fish you have now and wish for them to live their natural lifespans. For a betta, that is up to 5-7 years. I don't know about the other fish you have.
I'm glad you came with an open mind though- it's great you are willing to change some things about fish keeping. Keeping a cycled tank with the appropriate amount of fish is a very rewarding and relaxing hobby. I keep my 5g tank on my desk so when I'm working on school work I can look over at them and interact with them. I have my tank set up so it's pleasing to see, colors are complimentary to my fish's so it's still decorative and they have a suitable environment to thrive. You can do this too. IT will just take a bit of doing.
And you want horror stories? One of my tanks when I was a kid was made with those big blue water jugs people have in offices. The top was sawed off and I kept about 4 or 5 goldfish in it. My res turtled lived in a small stainless steel baking bowl for close to a year. My old 5 gallon tank had 3 gourami's, 2 cory doras, 2 rosy barbs, a male betta and at one point for a small period a dwarf puffer fish.