Did Charlie have casters? None, as units of Charlescomm. He would frequently hire casters from the Magic Kingdom for specific work. He had an excellent reputation as an employer there.
From summer update 46. Doesn't sound too very stingy to me.
I realize everybody's staked their positions by now, very much including me. I still think Jeftichew, tipped off by Charlie, meets Occam's razor the best. There's a carnymancer with a beef against GW kicking around the magic kingdom; Charlie's already hired his fellow caster from the same side and may know they've been asked directly to only work for royal sides (but with the strong implication of opposing GW); its suggested Charlie can intercept thinkagrams; and just after Parson briefs the plan by thinkagram, a caster who looks like a carnymancer happens by just as Parson is carrying out his (short notice) orders. I don't see the percentage in thinking its some other mystery player we haven't had introduced yet just happening by at that moment, or also having cracked GW's communications.
The exact relationship with Charlie is up in the air, could be anything from retainer since the time he hired the turnamancer, to a tip right when he found out, possibly with another bounty offer. It doesn't have to be full upkeep paid out in the real world, since barbarian casters apparently can support themselves in the MK.
I don't think its the thinkamancers, since they met in their temple and presumably came straight there after meeting. And are apparently paranoid about sending certain things in the clear.
You're right about the 600+ archons being a big deal. My guess is Charlie doesn't want to reveal how many he has, or even with numbers short of that reveal just how effective he could be at croaking a sides leadership, without even a contract, or ground support to get him in. He's made it this far by being useful, sometimes annoying, but never threatening to the royal sides. I'm not saying a direct assault by a coalition would go well for the assaulters, but just having people stop hiring him en masse would play havoc with his upkeep for a one-city side, and with his long-term strategy more generally.