I've played around with mixing salad greens all in the same bed
before and wasn't entirely pleased with the results. Black-seeded
simpson simply trumps all other lettuces in terms of flavor in my
opinion, so mixing in other lettuce varieties wasn't a hit. Meanwhile,
trying to interplant spinach was problematic since lettuce and spinach
grow quite differently.
But this year I randomly
sprinkled a thin line of arugula seeds down the middle of my last
lettuce bed before scattering the lettuce seeds across the entire
surface. Meanwhile, unbeknownst to me, some of the kale I'd let go to
seed in that bed had dropped propagules that were waiting for the proper
season in which to grow. The result? A perfect salad-in-a-bed --- 90%
lettuce, 3% red Russian kale, and 7% arugula. Yum!