While Trump has been soft on Russia publicly, the overall foreign policy establishment still remains hostile to them and they are still just as isolated economically and politically as they were during the Obama administration. As for China, regardless of Trump's personal affection for Xi, our policy toward them has been quite aggressive, in case you had forgotten we've been in a trade war for the last few years that shows little sign of letting up. Granted, this stance hasn't necessarily been very effective, if anything it has just made China retaliate against us just as strongly and encouraged them to expand their military presence to set up a buffer zone in the Pacific against the US Navy. However, I don't really see how Clinton would have had any easier of a time. The neoliberal consensus seems to be that she would've just been able to get the EU, UK, Japan, Australia, etc. to all just gang up on China and isolate them the same way that they did to Russia, but given that the Western world can't seem to agree on much of anything these days and that most of these countries are far more dependent economically on trade with China than they ever were with Russia, I'm not really seeing that being an effective strategy.
As for hegemonies, not all countries play power politics in the same way. America is rather unique because, despite our geographic location, our political alignment has always seen us look back toward the Old World, and events in Europe and Asia have always held far more weight than what's going on in our own backyard. This means that we've always been able to operate at a somewhat safe distance, giving us a free hand to meddle in other countries' affairs without fear of immediate blowback. If Russia and China decide they want to go strong arm some Muslims into submission, it tends to blow up in their faces almost immediately, witness the disasters in Chechnya or Xinjiang. When the USA does it, we can just drone strike some weddings and be assured that the retaliation will mostly fall on our local allies and few Americans will be directly harmed. Because of this crucial factor, America has been FAR more adventurous in pushing the envelope geopolitically than Russia or China could ever afford to be. The Soviet Union did try to do some of this during the Cold War, exemplified by the Cuban missile crisis, but given their vastly inferior resources they could never really compete with the USA as genuine equal, whereas China has never really ventured beyond their own backyard militarily and doesn't have the means to do so anytime soon.
Because of this, I'm not really afraid that Russia or China will step into the void if America steps down from its position as the sole global superpower. Rather, what I foresee is more of a multi-polar world, where the US, Russia, China, the EU, UK, India, Japan, etc. all kind of jockey for power and position, but with no single party sufficiently strong to singlehandedly dominate the rest the way that America does now. What this means is that we will settle into a sort of equilibrium where any country that gets too aggressive will cause a coalition to form in response to gang up on them and put them back in their place, but then return to squabbling among themselves immediately afterwards, restoring the overall balance of power. This is historically how the world has always operated, the current situation that came about after the fall of the USSR is rather unique and has given us a seriously inflated ego about our place and position and has led us to make monumentally bad decisions that have backfired spectacularly.