IMO if you own small caps, but no small cap index, you are OK investment wise, assuming a) you own between 50-200 small cap stocks (or a mutual fund which owns 50-200 small caps) b) your other positions in mid caps and large caps and foreign stocks are also diversified
I first started investing in 1997. Between 1997 and 1999 I did significant reading. The reading I did at that time suggested that the bull market run up and tech stock boom (50-100% returns for multiple years) and the index returns could generally track their returns to a small batch of stocks in the index. Meaning that if you look at S&P 500 returns 1990-1999 for example, I understood that much of the return was due to Microsoft, Cisco, Dell and a handful of many other tech companies.
So while an index invests in several hundred companies, its very possible that most of its return will come from a small percentage of stocks in the index.
My summary is then to be diversified in your positions (you do not need to hold an index fund for every market cap or sector out there) and follow what is going on with your investments 1-2X per year (meaning track performance and rebalance).