Ideally you want a finished hole size of 20.07 to 20.11 mm. These will allow a 20 mm peg (and all Festool accessories) to slide into the hole. 20.2 mm is rather large and I think he got those numbers from the last run of the MFT-1080. The last years those were made they had holes of 20.17 mm. I have one of them tables and the holes are definitely larger than need be.
While I can find no proof that loose fitting dogs are more accurate, I do believe tight fitting dogs accomplish nothing but grief for the user. Allow some slack for the dogs to slip in and out and using them will be more enjoyable. When the dogs are used properly, some slack will not affect accuracy at all.