Ah, you poor thing 
Right now, you'll want to poke both their eyes out. So my post might be too early for you, apologies if it upsets you.
I don't think there's much point in trying to influence when your daughter meets her - it will be his choice. If it will upset your daughter and he's reasonable, it's worth asking him to delay the meeting. But accept that you can't control this.
I also don't think it means there'll be an endless stream of people traipsing through her life - thank goodness!
This is the bit that's probably too soon for you to hear, but how I've coped is that I truly believe that a child can't have too many people in their lives that care for them, and look out for them.
Right now, it's SHIT. But it has to be better than your child one day enjoys having two families, than not.
My situation was far easier - my husband cheated, but with prostitutes, not this GF.
He's just been on a family holiday with them, my daughter (also 6!) chatters away about how it's like have a stepsister...
I wanted a second child, a sibling for her - his behaviour put paid to that. How is it fair that he gets to have the "family" and I don't? After his behaviour? It isn't fair.
But I can tell you this: my daughter likes both homes. She doesn't like them more than me. She's happy. And any time I feel jealous or angry of just sad, I look at her being happy. And I think - what makes me happiest in the whole world? My girl being happy and secure and loved. Her dad is a grade A arsehole - but they have a lo jdh little thing going on. You have to try, for your own sanity, the separate your feelings.
that you're dealing with it so quickly, after such a low marriage, and with an OW 