I agree with greenwillowherefordII. Your Char/Sim cross is comprised of two Continental breeds, and to have at least an optimal chance at GOOD cross breeding, a British bull should be utilIzed for Maximum heterosis (maximizing positive traits). But - the important factors which you should pay particular attention to are the QUALITY (both genotypic and phenotypic) factors - AND the functional traits of what the bull will bring to the mating. Good feet, good udders with small teats, Fertility, strong bone, capacity and easy fleshing ability. The Hereford has most of those traits, if you select him with those points in mind. So would an Angus or Shorthorn bull. AND - so would a solid Irish Black bull, as they possess ALL of the necessary traits mentioned above, plus EXCEPTIONAL marbling and minimal body fat and 13" - 16" rib eye - if you are thinking of a Terminal operation.
But I guess it is a moot question now, inasmuch as you said you were going to AI her today.
Sorry you were not early enough to get some advice before you bred her!
DOC HARRIS