Pretty sure that no. 6 is Romaine.

Looked again at 6 ... I think you're right, and I was getting thrown off because the plant immediately next to it to the inside is different, so:

6 = romaine, 6a = arugula.

I’m not seeing any Arugula. You could narrow the options by tasting them. There’s a marked difference between, for example, a lettuce and a mustard.

You have a terribly overcrowded lettuce mix. But it does look very nice!

No. 4 might be red oak leaf lettuce. When you look at it closely, does it resemble an oak leaf?

I think 2 is a red mustard.

Others say 6 is romaine, but I think it looks like it might be bok choi or tatsoi or something like that.

Just taste them! That'll settle a lot of queries. I’ll bet that 6 tastes of lettuce, not cabbage.

5 & 6 definitely look like lettuces to me. And I agree 2 looks like a red mustard. I think 3 is mizuna - I spy a light green version in there as well.

And tasting will tell you a lot, for sure.

You can pull them out, take a closeup pic, and ask about the ones you really like(or hate)

I agree 3 is red mizuna. I thought 6 looked like spinach - but could be bok choi - does it have a thick white base?

#6 looks like a Romaine/Cos lettuce to me...

There really isn’t any need for all this discussion and guess work. A few nibbles would sort them out immediately, but the OP seems to have moved on.