I make up all feeds at room temperature, and I will keep them for 2 hours, as with regular formula. I actually bend the rules a little on this one, because I will keep the milk out for up to 90 mins, then put in the fridge for up to 24 hours, heat and use again. The second use I will have it out up to 2 hours. We live in a hot climate too, and LO has never had a bad reaction to it. Not saying it's recommended, I'm just saying I do it and it seems to work fine.
If I'm reheating a bottle, I won't add any fresh milk to it, because my LO is notoriously fussy with her feeds; you can never tell how much she will have, so sometimes has 2oz and sometimes has 6oz. I therefore make 5oz bottles and refridgerate the leftovers. For that reason, I may end up with bottles with only 1oz left in them, and because once you reheat it you can't use it again, and I don't know how much she will drink, I use the old milk first, let her drain that bottle, then make some fresh if she still wants more. I don't think there's anything wrong with adding fresh milk to a reheated bottle, it's juzst that if your LO doesn't take it, you can't then chill that one, because the old milk is mixed in.
In regards to old and new cartons of formula, I will use all the formula from one carton, and if I need more to make up a bottle, I will take from the new carton. But each pack should be discarded within the recommended time frame (different for all brand, but I use Aptamil, and they say you should discard unused powder within 4wks).
