concrete forms: plywood thickness?

It sounds like you are forming pilasters in a gap in a stacked block wall? The problem on pours like this is, they usually get filled up all at once so that the concrete is exerting all of it's hydraulic pressure against the formwork as you run the vibrator in and out of that stack of mud. That will pillow the plywood that is between any bracing, jack the edges that are against the block wall and pull any kind of anchors you try to drill into those blocks. You will need through ties as close to the vertical edges as you can place them, and additional through ties in the field all depending on how wide and tall these pilaster are. When columns fail during a pour, they are usually pretty dramatic. Like a zipper from the bottom up.

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You do need the top of the forms to be at the exact height you want the finished slab surface to be at - The top of the form is exactly what the concrete finishers will use to determine the height of your slab's surface. They use screed boards, either a long straight 2x4, or a straight aluminim piece, to drag along the top of your form boards. They continue dragging the screed board until the concrete has worked its way down to being the same height as your forms.

Put another way, if you make your form boards higher than you want your slab, your slab will be higher than you want it

Should look like this, with the top of the gravel in the pic exactly 4 inches lower than the tops of your forms, so that the concrete all across the slab is 4 inches thick everywhere except for where its thicker in the footings:

In '92 me & my dad built the forms the monolithic slab for the 1,600 SF house I still live in. We used 2x12's & secured them to 2x4 stakes about every 4 or 5 feet.You do need the top of the forms to be at the exact height you want the finished slab surface to be at - The top of the form is exactly what the concrete finishers will use to determine the height of your slab's surface. They use screed boards, either a long straight 2x4, or a straight aluminim piece, to drag along the top of your form boards. They continue dragging the screed board until the concrete has worked its way down to being the same height as your forms.Put another way, if you make your form boards higher than you want your slab, your slab will be higher than you want itShould look like this, with the top of the gravel in the pic exactly 4 inches lower than the tops of your forms, so that the concrete all across the slab is 4 inches thick everywhere except for where its thicker in the footings:

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