Since I'm no expert, I've followed the MMM advice of putting my money in a total market index, so I am heavily invested in FSTMX.
Ok, good. But that means you
already own all the stocks that are included in FSSVX, NAESX, and FSEMX. That's why they call it "total market". :-) A small-cap or mid-cap fund is merely a subset of a total market fund. So adding a small-cap fund does nothing to increase your diversification. It just "overweights" you on small-cap companies (you would have proportionally more money in small-caps than a market-capitalization-weighted index does). That *can* be a valid approach (some research shows that overweighting small-caps has historically beaten a cap-weighted portfolio), but don't confuse it with diversification.
If you really want more diversification, then I would recommend an international index fund instead of a small-cap fund. Because, here's where the "total market" name
is actually misleading: it stupidly refers only to the total
US market. So if you add something like FSGUX, then you will definitely increase your diversification, and essentially own all the public companies in the world.
See
http://www.bogleheads.org/wiki/Three-fund_portfolio