Selecting a SSD for Fusion Drive
It is pretty close to "full capacity" (the first 117 of 128 GB are filled), but SSDs do overprovisioning for this reason.
As long as you have enabled TRIM on your SSD, you should be fine. I keep my (non-Fusioned Crucial RealSSD 300) SSD near full capacity and don't experience degradation, as long as I have TRIM turned on (if it's turned off it gets terrible fast).
Yes, you can create a Fusion Drive by specifying the partitions, e.g.:

diskutil cs create fusion /dev/disk0s2 /dev/disk1s2


If you are looking for non-Sandforce controlled devices, you have many options now: Crucials M4/M500, Samsung SSDs, OCZ Agility/Vertex 4, and more.

For further reading, check out AnandTech's article with more details and pretty graphs.