I assume you are going to trellis the cucs. The cuc on the south side, if trellised will shade out the squares on the back of it. Maybe you wanted this to happen to provide shade for the spinach.
What kind of summer squash. If it is yellow squash, there are very few vining varieties. If it is not vining, it will require a lot more than one square. The recommendation is one yellow summer squash per 9 sq ft. But since you have it on the corner, it will not require 9 sq ft. But surely a lot more than one square.

I was actually going to guide the (non-vining) cukes and squash off the bed onto the ground around it (which is why they're at the 4 corners), so it won't choke off other space or require a trellis. Because, as you mentioned, they're aggressive growers...