General view was that the USSR wasn't any friend of the West's but that Nazi Germany and its allies were even worse, plus Germany's in a big position to be setting up right on Britain's doorstep (i.e. Belgium) which is historically going to make Britain very, very hostile to any power that has done or is capable of doing that. So it is an issue of geography as well too, sure the Soviets are scary but they're far away, are you more scared of the big, burly ex-con who lives in a different city or who's your next door neighbor? Yes it's a silly analogy but that's kind of how it was foreign policy wise.
Also it's not just Russian, yes I know that's the common term but it's better to say Soviet, otherwise you exclude all the Belorussians, Ukrainians, Kazakhs, Kyrgyz, Tajiks, Georgians... you get my point, who were all part of the Soviet Union.
Also...
How does the US enter this hypothetical war with Japan being on the Allied side against the Soviet Union? This may well mean no Pearl Harbor since Japan has bigger fish to fry and can get the resources it needs (and that motivated the Pearl Harbor attack) elsewhere.
As for the likelihood well I did go over it but overall I'd say kinda unlikely, a lot of AH assumes really easy alliances of convenience (and sometimes that's justified) but in WWII's case it would require huge provocative action that established the Soviet Union as basically as big of a threat as OTL Nazi Germany was thought of as and that's just not in the cards for Soviet leadership at this time, Stalin was looking to reclaim historical Russian possessions and strengthen the USSR's place in the world but he wasn't about to go about doing it in a way that would turn everyone against them.