And it would be fine if we did that, gives us flexibility. But ask yourself what Allegri has said his formation is and he wanna stick with, 433. So in that sense, it makes way more sense to first sign players for that main formation, before you consider a player for your backup formations in terms of flexibility. Especially with limited funds.
And Cuadrado works in all formations because he is a fullback too, so he can be kept around. But for Kostic, most likely as Igor said, he would only work as a wingback in 3421 or maybe in 442 as you suggested. But aslong as those wont be Allegri's main formation, a deal for him are unlikely. Would been easier if he could fill in as fullback too in 4 man defence, but seems like thats never been the case.