I don't know about cedar trees, but it does take 2 concreet blocks per Christmas tree with the needles still on them. I tried 1 block per tree years ago and had a line of tree tops just above the surface when I got done, so had to add another block to each to get to go down.
I've heard that it is best to let the needles fall off Christmas trees before sinking them, don't know about cedar.
This early spring the KDWP put 100's of fresh cut cedars in Clinton, so it must be okay to use fresh cut cedars, but I'm not sure how long it takes the crappie to hang around them.
I might suggest wrapping some copper wire around the trunk and putting some in the block cavity before filling with concrete to keep the trunks in the blocks.