My current design for a garage is 24' by 24'. There are 2 garage doors on the front 9' in width, with 1 1/2" on each side to install them into. This leaves 66" for the front facing CMU wall, which is split into three sections of 22" each. These sections are thus slightly shorter than a standard CMU, mortar joint, and a half sized CMU (23 5/8"). How are situations like this handled? Do masons cut down a solid 6" by 8" by 8" to 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 & 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 & 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.