plonz 

This extract from the second link states "And the fourth stipulation is in regards to 6 and 7-speed HG-style cassettes and freehub bodies. Both 6 and 7-speed HG cassettes are narrower in width and therefore can be installed on any of the above HG freehub bodies with the addition of a 4.5 mm spacer behind the cassette. However, newer 8, 9, 10, 11 and 12-speed cassettes cannot be fitted to old and narrower 7-speed freehub bodies." This indicates a CS-7800 10 speed cassette does not fit on a 7400 freehub. I think OP intends to remove an inner cog make it a 9 speed so maybe it will work on a 7 speed freehub. Guessing this will come down to trial and error.