the spacing can be found under the faq tab between galleries
and tools & resources.
if you want more info it can be found in the square foot books.
ttfn

Peppers work just fine for me in 1/square.

I grow my indeterminate tomatoes in a different garden bed -- generally 4 plants in a 4'x4' raised bed, with lots of compost, and they get huge (especially the cherries). This year, I am planning to try a couple determinate tomato varieties in my SFG, 1/square, because they stay smaller and shouldn't need to be pruned so aggressively to keep them in bounds. But the squares around them will be filled with things like spinach, carrots, beets, and lettuce, which can be easily harvested if/when the tomatoes need the extra space.

Cucumbers in the SFG will be new for me this year -- I'm planning on just 2/square, on a support, and will see how it works.

My personal spacing preferences:
Peppers 1 ft apart. They like to rub shoulders.
Indeterminate Toms, which I string, 1 1/2' spacing for better air circulation. The other 6" of space in front I put lettuce, radishes, and spinach like kathy mentioned above. They are harvested before the tomatoes overrun them.
Determinate Toms I grow in big pots with tomato cages. They are limited in size, some only 2' tall, others about 5' tall, and produce all at one time. Once they have their flush of harvest, they give very little else.
Cukes 6" apart on trellis or with tomato cages.
Melons, smaller ones including watermelon, in a raised berm (no boards around it) so they can spread in all directions without interfering with my other crops.
You will find your niche and what works best for you in your micro-climate, zone, frost dates, and space.
Good luck.

quiltbea, is that styrofoam under the watermelon?

Yes mistigardens, I recycled some packing materials which included foam sheets. Putting them beneath the melons kept them from getting too wet and rotting.....plus made them easier to find.

wonderful idea, thanks for sharing!