For 1: Typically I don't Need if there is another mage in the group that is obviously leveling up their class and not level synced. If something drops that I want I will ask if it's ok to Need on it. You can also just ask ahead of time when the run starts.

2: Your gear gets capped to what gear would be at the level you are at. So Level 24 cap sets your gear at item level 24. And yes it is much, much better to use lower level gear in these dungeons because of repair costs and the fact that you get no benefit by using the high level gear anyways.