I was thinking about dropping down from 5x10-12 on my assistance lifts to 5x6-8, mainly on flat and incline DB bench and any type of rows. I'd like to put on some more size but most importantly add power so I can increase my main lifts. Let me know what you guys think.
That will be fine, after you try it if you don't like it, up the reps again. You won't do any harm doing 5x6-8 for your assistance. What I would suggest is slowly upping the weight and slowly lowering the reps until you get to that point, but that is just me.
It won't hurt to try. If you bench twice a week, then I'd go sets of 5-8 on your heavier bench day.
I like doing a couple sets of 5 with approximately 75-80% of my max for the day (or approximate max based on whatever I've done that day) after my "max effort" bench work. For instance, recently I worked up to a triple with 305 on floor press and then did 3 sets of 5 with 275 on decline. Since I have a pretty good feel for how my numbers on various lifts relate to each other (this entirely personal, too), it's not hard for me to make an on the fly decision about what weight to use for my sets based on my performance that particular day.
I don't like going to failure on assistance work.
Damn... You do tons of sets. On assistance stuff I do like 3 sets including "warm ups".