I'm not sure about their relationship with the Syndicate, aside from working in the same area. It may be as simple as a place to sell their wares.
How much are the Angels involved? What about the deal between the Angels and the Serps? Obviously the Serps have some muscle - they stole a titan, after all - but how much do they depend on the Angels? How closely are the Guardian Angels still tied to their parent organization and how closely are the tied to the Serps?
As for the Angel/Serpentis relationship, I've done a bit of reading and piecing together what I can from the missions out there and the PF from chrons and the wiki; and this is what seems to make sense from what I've seen.
The Serps have a very minimal amount of muscle, and what they do have is dedicated to defending the drug manufacturing plants and storage facilities. Even what they have for those tasks isn't enough, which is why they hire the Guardian Angels from the Angel Cartel to be their muscle (you'll even note that the Vindi, Vigilant, and Daredevil are actually Guardian Angel ships, not Serpentis ships).
The Cartel sees this as a mutually beneficial arrangement, as they get first pick of the boosters that the Sarpati family produces and of course there's the racketeering view as well (the Serps would be screwed without the Cartel, the Feddies would have wiped them out the way I understand things; and if they weren't hiring the Guardian Angels, who's to say they wouldn't).