I am aware that the other Kings wouldn't care about us hiring the Fearsome Five, well, other than thinking that we are nuts for willing associating with people with a connection to Negaduck. The issue is that the Fearsome Five don't really seem, well, discrete. Doom would certainly take advantage. Doom is going to be back in the Intrigue game soon, since he did kill Hawk. Furthermore, what would Doof think? Why would he do something that risks bringing a member of the Fearsome Five to Doofania, where Vanessa lives.
No, what I meant is that the other Kings wouldn't know that we hired them, the same way we don't know who they hire and if the other Kings don't know the general public wouldn't too.
Though I agree that they aren't discrete and that they'd probably have quite some problems with some of our Hero Units, I don't think this is unmanageable (like, look at Jumba for example).
Basically, what I'm trying to argue is that, while hiring one of them wouldn't be great it wouldn't actually put us in a worse position by just hiring them.