If you import a character from ME2, if you destroyed the heritics, can you still have the Geth and Quarians become allies if your paragon/rep is at the max level?
Also, related to the above, but if you make a character in the game, and your paragon is high enough, can you still make this alliance? I'm talking about making a brand new character, with nothing carrying over from the second game.
Any help would be appreciated.
If you make you character new in ME3 it's impossible. One of the requirements is that Tail is present, and if you started new shes is exiled and won't be there as an Admiral.
As for your first question, I'm pretty sure it doesn't matter what you did during Legion's loyalty mission, as long as you did it.
After looking at the wiki, it doesn't seem to matter if you rewrote them or destroyed them. If you scroll down to the yellow word that says Note, it should tell you everything you need,
It seems to matter only that Legion was activated,loyal and survived the suicide mission
and it doesn't work unless Tali is an Admiral..and I think you have to actually rescue that other admiral too
I have done this twice. I got them to shake hands both times. Tali was exiled in one, but I made the right choices. You need to disable the Geth Fighters and you need to save the Admiral. If you have high Rep you should be able to do it. The choice to Rewrite or Destroy only affects the War Asset. If you Destroy, the Geth have less WA and the Quarians have more. If you rewrite, the Geth have more and the Quarians less.