Medakas are nice little fish, and will live in a temperature range from 3 to 40 degrees.
Tho I've never had a problem with my rainbows (sorry don't know what type, it was years ago I first got them) or pacific blue eyes with temperature.
Winter will sieve some of them out, but they breed so readily it's usually the weak that go.
Your danger area will be summer, as you get pretty hot up there. Especially if the bin is sitting on concrete.
But if you have lots of rocks and pot plants around your pond that will keep it a bit cooler. You can water around the pots and rocks to keep them cool when it gets really hot.
And a 100 litre will be fine as long as you don't overstock it to begin with. Probably a good idea to get your plants established a bit then get your fish. As I said in other post, water reeds and bacopa are easy and will grow readily.
You have a lady who breeds medakas up your way, she has black, gold, white and orange.