When to let cats be together?

Sounds like you've got it under control!  I would keep taking my time but if everything looked and felt "right" to me after letting them play for a while through the gate, I'd take it down and see what happens.  I'd keep them separated while unsupervised until the sessions where they're together seem solid.  I would start with the cats only and then add the dogs later. 

If they are already on friendly terms, which it sounds like they are, you can introduce them right now. The main thing is, you don't want to leave them alone in the same room until you are pretty sure they have worked out all their issues. Hissing is pefectly normal and to be expected, especially from Poe as the resident cat, and so is a bit of roughhousing, but serious fights should be broken up and the cats should be seperated for a while after that. If they do start fighting, do not yell at either one, and do not pick up either one. The one you pick up will be seen as your favorite, and that might cause behavioral problems in the cat you didn't "rescue."

Stay calm when you do the introduction and both cats will feed off your confidence and should be fine. Also, make sure to go out of your way to show Poe at least as much attention as Ginger, or Poe may resent Ginger and start fighting with her. Cats are very territorial, and if you support Ginger over Poe, Poe will think you're picking favorites. Don't worry if they are cold toward each other at first, female cats tend to be more territorial than males. As long as the cats aren't fighting, you should be fine leaving them alone together.