The HF-500 can support up to 8mm ER11 collets.  The most popular in North America are 1/8” and 1/4”, but they have collets that range from 1mm to 8mm that will work.   The ER11 is an industry standard that determines the overall collet size.  ER11, ER16, ER20, ER25 are all common sizes.   But the HF 500 and our Tool Changer tool holders for the MM-1000 are both designed for the ER11.