I think that your plan is fine =) I'd make sure you have some plants in the 2.5 gallon... I've never really moved pregnant guppies because everytime I do, I have bad luck with it. I think you'd probably need a heater but that just depends. I don't have a heater to actually keep the water warm, but I have it instead to keep it from changing too much at night, on hotter days, etc.