Most modern day forks will come with a built in race holder.So with the added thickness of the cup wall that needs to be inserted into the headtube, the space will be too small to accommodate most new school forks.Obviously, if you are using older style forks (kinda rare by todays standards) and a non integrated frame (again not a common sight by todays standards in flatland or Bmx) then it should all be fine.But for anyone who simply hopes to use the headset on an older frame with newer parts they could run into a host of different issues.
Re forks i have Odyssey Directors on my 20" (built in race) and Landing Gear on my 24" (no built in race) and both work just fine