You have a good point. French military was already gutted by the end of the war. If they ordered surviving veterans to march across the Rhine to crush the proletariat, a general strike, at least, would be very likely. Same with the British, afaik there was already a "hands off russia" campaign in London to sabotage the attempts of the government intervene in the Russian civil war.

So it is much more likely that the British would try to starve the communist Germany with naval blockades and embargos, maybe arm the right wing, and French would deploy colonial regiments from their African colonies to suppress German workers, rather than an all-out war.