It's the scales in the province where you cast it that counts. After about 6 casts in a Luck-3 province you end up with some pretty funky stuff, like size 6 monsters, fliers, along with a load of chaff. I think it's fun, but I didn't use it in MP.
Sombre's amazing Skaven mods Cross Breeding like this
Code:
#newspell^M
#copyspell "cross breeding"^M
#name "Moulder Pitbreeding"^M
#descr "Pitbreeding ... (snip)" ^M
#school 6^M
#restricted 73^M
#researchlevel 2^M
#path 0 7^M
#path 1 3^M
#pathlevel 0 1^M
#pathlevel 1 1^M
#fatiguecost 1000^M
#end^M
It changes the casting cost, paths, research level, etc, but not the result.