Since I have run up to 60% nitro in custom mix fuel... the 25% is obviously not out of line. (at that level of nitro... the methanol is a coolant, not the primary fuel) don't go lean with super-high nitro... you melt pistons.

You can get 30% nitro fuel off the shelf...

As for flying in the cold... If you can stand it... the airplane really doesn't care. You have to limit the cooling of the engine with baffles... and might need an on-board glow system to keep it running. (really cold air can put out the glow)

I've flown in rain, snow (while falling) sleet and freezing rain. (had to land due to icing on the wings once...)