Why not? Defeat ISIS and deal with the differences after, hopefully diplomatically. I agree with Trump - I rather be friends with Russia or Iran than the last 30-50 years of hostility that hasn't got us anything but more enemies.
Also it's not a full alliance: it's just an alliance against a common enemy. Once that's cleaned up, work on the future of trade and friendship and not further military alliances.
Because he also says dealing with Iran is the worst thing in world history. It's the nonsense. Of course everyone wishes we could all get along.
And Russia is not targeting ISIS. Neither is Assad, really. They're targeting the other rebels, and ISIS will be the last standing because if they take out ISIS first then the rest of the world will tell them to stop.
