In order to get more hight in my loft conversion is it ok to cut through the first floor ceiling joists so that the rsj sit lower ,then strap the ceiling joists back up to the new loft floor joists? aprox 200mm(width) needs to be cut, the first floor ceiling will be supported on acro's whilst this is done, the existing ceiling joists and the new floor will run parrallel with each other, Ill also need to fire proof the existing ceiling joists with rockwool over chicken wire, is it acceptable to compress the rockwool      

It's difficult to give an opinion on this without seeing the layout or the drawings.

Why not ask whoever drew the original plans?

Well as far as I can understand this, it all sounds as if you could lower this rsj as you want.  You are not changing any of the loadings are you?

It is essential that you fix all the cut timbers back to the rsj properly.  Often the best way is to shot-fire timber infills to the webs of the steel and then 'Jiffy' the joists onto this timber infill. The Jiffy straps should be long enough to wrap over the steel and be shot-fired into it on the top.

Don't forget though, if you are cutting a section out of the existing ceiling joists then the strapping effect that these joists have on the roof will be lost (at least temporarily).  This could cause your existing roof to spread (rafter feet move outwards) so you must take all possible precautions to stop this happening.