That's the beauty of raised beds (or any beds for that matter) is you can intercrop. Since you will not be walking in the beds, you just have to make sure you can reach your crops from either side. Now, for the toms, that would probably mean planting them closer to the edge.
But as far as too close goes, it all depends. Typically, I've heard of square foot gardeners giving toms 1 sq. ft. each. this translates to about 1ft. equidistant. for the cukes and squash, it would depend on whether the squash was bush or vining and whether or not cukes were trellised. As Apps stated, going vertical goes a long way in getting them maximum use out of a space.
For the lettuce, you could actually plant that in-between your tomato plants since its root system will not be directly competing with them. In fact, this would be beneficial to the lettuce in that the tomatoes would shade the lettuce and help keep it cool in the summer.
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