I don't think Carrion will join Clayman, per se. Carrion respects strength, as is shown by his subordinate's interaction with Rimuru's subordinates. The one who completely showed him her incredible strength is Milim. If he is going to join anyone, it will be Milim, not Clayman. Again, I don't think Milim is brainwashed or being controlled, since she is way too powerful for something like that to happen. However, I do believe she is being manipulated, which is Clayman's specialty.
And at the end of the day, I don't believe Milim will cause direct trouble for Rimuru (indirect in that a flood of refugees were forced to come to Tempest). Even if she does, Rimuru now has Veldora, who I'm guessing is probably her equal. We don't know how strong Diablo is, so I'm going to hold back judgement on if he can handle Milim or not.