The druze are also very secretive and don't let a ton of outsiders in. I believe that all of the shock team members have druze lineage so that probably doesn't help with their recruitment lol.
Theoretical it would but practically I think Saito does fill his job already (and even more so with the Specialist upgrade). Guess silent ninja killer fits even better then psycho killer, who also works a lot for the "good" guys.
There are too many disagreements between StarCo/Corregidor and Druze fluff wise. Therefore he doesn't fit into them
Working for Druze as well as Corregidor/StarCo would clash with his Aristeia! appearances. They also don't appreciate his sense of humour.