To raise your CYA quickly enough to get "some" in the pool, go ahead and use straight CYA--I would target about 15-20 ppm when figuring how much to add. Then you can use the trichlor from there until it gets to where you want it. As far as pH goes, the trichlor WILL bring it down, and 7.8 isn't too bad to begin with so I wouldn't worry about that.

Janet