You might want to read up more about the different schools of thought around investing. The big key I see is making sure you define an asset allocation that makes sense, then deciding which funds get you there.
I'd highly recommend resources like Jim Collin's Stock Series
http://jlcollinsnh.com/stock-series/ and the Bogleheads Wiki
https://www.bogleheads.org/wiki/Main_Page Tons of great books out there, too, like A Random Walk Down Wall Street and The Little Book of Common Sense Investing.
For me, my allocation in US stocks follows the total stock market, which is biased more towards the large-cap/S&P 500 companies. So for me, I would have more of the Black Rock S&P 500 fund than the Fidelity Extended Market if those were my only options.
"Extended Market" is essentially "Total Market minus S&P 500." If you would prefer to invest in something like VTSAX rather than slicing and dicing, buying the S&P 500 fund and Extended Market fund in an 80/20 ratio will basically get you to that point.
Yep! That's what I would do for my plan.
Edit: Mixed up the original funds in my first post!