You are correct. It's not always in a straight line.
I think once Trump actually comes into office, we may hear a different tune on his stance concerning Russia. But while I am no fan of the man, I do realize that he is reasonably intelligent even if he needs to have his Twitter account taken away from him. For a great power such as Russia, military intervention on the scale of their involvement in Syria is not sustainable. So while Trump may openly support their intervention to help out the Assad regime, in reality it's just another way to drain Russia economic resources. Russia is a tremendous military power in comparison to most countries, but they do not have the economy to sustain military operations in the same way the United States can.
The truth is that the Russian military intervention in Syria is a political gamble to gain legitimacy in Middle East region. If successful, this could pay off politically and economically for Russia for many years to come. But they're up against significant challenges, and are opposing a world superpower who has a military and economy many times their size. Like the United States, Russia has joined the great "let's-herd-cats-race" that is the Middle East, and we'll see if they have the resources to follow it through.
As for Iran, if you really want to understand the US position against them, just read:
https://en.wikipedia.org/wiki/Saudi_Arabia–United_States_relations