You can never have enough...

[Oh_hunnihunni]

... two minnows and any Endler escapees ...

The minnows are fine outside all year - they are cold water fish. the endlers might be a different story though. They are so closely related to guppies that I would expect the same temperature tolerance, which is that 15deg is OK but by 12deg they are dead. So nights might be a bit borderline unless you can run a heater. They are small enough though that a few in 4 litre jar inside would probably be fine.

They'll be fine. They've spent a full year in my cool water tank which got down to 9 degrees over the winter, just not for long. The just slow their metabolism right back to survive the odd short periods, and being heavily planted with a lot of rock and wood to act as a heat sink helps as well. I have found the Endlers much more resilient to various environmental changes than guppies, especially the overbred hybrids. Being a pure strain might just work in their favour. If my 'tropical' ramshorns can survive outside over winter, I think the fish have a good chance.