You mean the entitled heretic Von Strab? I thought that he tried to boss around the Astartes but was cut short by Dante. As far as I know, the Astartes answer to no-one but the High lords (and submit to your command only if it serves the Imperium better). And the Titan force was bound by some form of oath rather than a standard protocol. Mechanicum generally doesn't like interfering with their structure and enjoys unprecedented level of independence. You can basically reach an agreement or form an alliance with a particular Forge world or Legio Titanicus but otherwise they are bound by nothing if it's not in their own interest. And just as you said The Imperium is more of a network of alliances and every world is kinda unique so it warries a lot in terms of the planetary governor's level of command over various aspects of Imperium's local resources. In most cases, they have the Arbites, the PDF and most likely the local IG regiments (though the segmentum command still outvotes them if they choose so).