I drilled out the 4.0mm centres of some Cyclone/Spectre magazines to fit the 4.3mm FX2000, and after I had drilled them I tried them back in the Cyclone where they cycled and shot without a problem, although obviously the correct magazines for each rifle should always be used where possible.
The 7 shot mag was an RWS Excalibre quirk, and as far as I am aware, the 8 shot magazines work perfectly.
I can't tell you what the magazine centres are on the Excalibre as I've never owned one.