They would certainly be a threat to Guderian, but only if he tried to grab power. Why should he be doing so in the first place? Goering and Himmler are military laymen, and at least most of the time they are not deluded enough to deny it. Generals do not grow on trees, and the top Nazis, whoever they might be, need the generals. They have to execute some of them because of their involvement in von Stauffenberg's conspiracy, but it would be absurd to execute Guderian (provided Guderian does not want the leadership for himself), who is both competent and reliably pro-Nazi. So Guderian is much safer from Goering or Himmler when not trying to be the top guy in Nazi Germany.
There is another reason why a coup attempt might not look at all attractive to Guderian. He is, in all probability, able to see that Germany is going to lose the war. As Inspector General of the Armoured Troops or as Chief of the General Staff he is a soldier and enjoys the protection of the Geneva Convention. In OTL he was never tried for any war crimes. But as the head of the German government he will in all probability be held accountable for crimes that the Germans committ under his leadership, which might very well cost him his head. Why take this risk for the very dubious honour of ruling a country in ruins for a year or so?
Not supporting one side in a country's power struggle is not evidence of wanting power for oneself. I am perfectly willing to take back what I have written so far, if real evidence for a coup attempt by Guderian, or serious considerations for the same, can be found in OTL, but so far this has not been shown in this thread.
People with whom you have good long term working relationships are not neccessarily fellow conspirators. The former might be turned into the latter, but this might equally well backfire, when your so-far-loyal co-workers report your conspirational plans to the relevant security services.