How to build a CMU wall to specific size?
Yep, you cut them just like bricks...with a brick hammer. Just score around the block with a chisel or the hammer's straight claw &amp; it either pops right off or just needs a whack. You'll might have to break a few to get it right, because they don't always break clean since they aren't as fine a material as brick.

But, there's no problem in chiseling them shorter or cleaning up the cut to fit.
And yes, you can also mix in Solid CMU's or just portions of them. That's why they're called Concrete Masonry Units &amp; not just the single type of "Cinder Block" anymore, there's a large variety of CMU's. Really whatever works is right as long as it's tight.