Not sure about compatability of the two fluids. Personally, I would try to drain as much ATF out as possible, then flush with clean steering fluid, and run only steering fluid from then on.
I don't know what the filter change interval is for the steering. An empty system will take nearly two quarts of fluid, an empty resevoir about 2/3 to 3/4 quart (at least on my 108, which is fitted with resevoir #116 460 01 83).
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.