In Serbian(cyrillic) we have nj, lj, dj, tj as one letter, and here it is nj, lj, dj, tj.
It could be the same with sx, zx. It could be sj, zj.
And cx, gx could be tsj, dzj, since it is pronounced that way.(try saying t and sx together) It could be that way if most of the people agree, but I think it shouldn`t be. Slovio was created to be simple and easy to understand by Slavs. Since all Slav languages use Z, which is not the case with X, I think there will be less confusion using X. When some of you say that a person wouldn`t know how to pronounce CX you are not considering that if that person doesn`t know Slovio the word doesn`t make any sense to him/her anyway, but if that person has learned Slovio, then it wouldn`t have been hard for him/her to learn how CX is pronounced. If you don`t like CX because it doesn`t exist in any Slav language, than think of the (I)S for plural and (U)F for direction.