The real answer is midcaps are covered by a lot of index funds. many people consider the company i work for to be a midcap and it is larger than many companies in the s&p500. Also many small cap funds cover midcaps. Or a total market fund covers midcaps according to market cap.
The OP is wise to ask to ask about midcaps, but should use tools such as Morningstar‘s instant xray to see if their portfolio already covers them.
"If you only have 1 year to live, move to Penn...as it will seem like an eternity."
Good to see you back, I was starting to miss your incisive commentary!