The 10g is just about right for a baby. Just make sure there is NO open space by putting in greenery and such.

The largest tank a BP should ever be in as an adult would be a 40g long. And, you'll have to wait at least a year or so before moving the snake into the 30 you say you already have.

That 75 would better house fish or a cool lizard... or a boa.

Make sure, with any tank you use, that you cover and insulate the back and both sides, cover the screen top 3/4 with foil or plastic, and use a good thermostat with an Under tank heating source for heat. Good luck with everything for the little one!