K...visualize the finished size of these veggies. It would be best not to crowd them together. Sometimes less is more. Go for quality...

I have never harvested nor grown the salad greens; I've also never grown radicchio. As a result I have only a vague idea of how big they're going to get.

One variety of radicchio I have can be spaced closely together, and the reason I'm toying with the idea of the Chinese cabbage is cause it's a 'mini' variety.

You can grow salad greens pretty close. They'll only really be an issue if you're planning on growing full heads.

I tend to harvest salad the whole season (pulling off a couple leaves from each plant), so packing them close never gets to be too much of an issue for me.

I'm growing mine hydroponically though, in 3.5" netpots, in gutters, with the netpots right next to each other.. i've got more than one plant in some of the pots, and they all seem to be growing fine. Its mostly Romaine though, which tends to be more vertical than some of the other lettuces.

Frankly, for me, its not worth setting aside a 10" pot for the season when its only going to get me a $2 head of lettuce. Better off growing peppers or something thats going to produce multiple fruit over the season.