Ok, I know it´s a unlikely possibility, but given Starscream actual situation (alone, without allies nor resources and both the autobots and Megs wanting his head) an alliance similar to Arachnid´s with Mech may not be a bad plan for him. Surely, he despises humans, but after being ready to kiss Optimus´ butt to save his skin, his dignity could tolerate to become an ally (not subordinate) to Silas. After all, without Soudwave´s assistance may be very difficult to him to locate new hidden decepticons warriors in stasis, and no garanties to be more receptives to his cause than, for example, Skyquake (and that time he was at least nominally in charge). And even if he could raise a bunch of terrorcons, they are too mindless to be really useful without backup. In respect to MECH, no doubt Megatron´s ex-second in command´s help would be unvaluable to his cause, not mention his knowledge about cybertronian technology or even lend the body of any captured vehicon to dissect. As long as they could restrain their mutual betrayal urges, it could even work.