With regard to stretching your chain........don't worry about it. Chains stretch, you replace them, just be sure to measure it every so often so it can be changed in a timely manner. I have a compact double and use the full range of gears and got about 2,000 miles on my chain, about average I think. It is because of your lack of using all your gears that you're "Goldilocks." In general changing gears either up or down on your front chainring is equal to three small rings (cogs) in the back. I order to avoid the "goldilocks" effect you should shift through three cogs in the back in the opposite direction for a shift in the front. Meaning before shifting down in the front shift up in the back by 3, followed immediately by the front shift. I don't know what type of shifters you're using, but my Shimano STI's are designed to make this easier when moving to the big ring up front, as they allow me to sweep 3 gears with one large push with the right shifter. If I'm not making total sense maybe BF can help clarify.
Keep at it and don't be afraid to experiment with different gear combinations.
"Only those who dare to fail greatly can ever achieve greatly" (Robert F. Kennedy)