Offer your insights or judgment on the input financial query or topic using your financial expertise. Reply as normal question answering

Buy small-cap ETF when you already have large-cap of the same market
Yes, you should own a diverse mix of company sizes to be well diversified. While both will probably get hit in a recession, different economies suit different sized companies very differently in many cases, and this diversity positions you best to not only not miss out in cases where small companies do better out of recessions than large, but also in environments where small companies rate of growth is larger in bull markets.