You say it's not changing down well at the front- one thing you can do if it's just struggling a little to move down is to depress the lever slightly beyond the point to allow the extra bit to allow it to move not enough to make it shift down the extra gear. Not sure I'm good at explaining that.
And yes as you have worked out avoid big big or small small. When I got my first bike I was told by the shop owner, that I could use gears 1-4 when on the smallest ring, I think it was 2-6 on the middle ring, and 5-7 on the big ring. I would say listen for the rubbing sound and work out which combinations work on your bike.