*Were there any concerns that San Marino could be used as base for communist infiltration into Italy and the West?
*Were there any hostile attempts to topple the San Marino communist government like what happened in other countries that had left-leaning government in power?
Edit: I found this old newspaper from 1951 Chicago Tribune newspaper reporting that [Italy blockaded San Marino to persuade the microstate to hold an election] (http://archives.chicagotribune.com/1951/06/11/#page/40/article/sanmarino-near-ruin-in-cold-war-with-italy). How did the US and her other allies viewed communist-ruled San Marino though?
What does communist government mean in this instance? The Popular Front in the Second Spanish republic was a coalition of left leaning parties (communists being a significant part of it) and was democratically elected in 1936.
Well, in a certain sense neither the coalition governments such as the Sanmarinese one are socialist or communist.
Point here is, the San Marino government from 1945-1957 was
not a communist government, but a left-wing government with communist in it;
not the first: without going back to the Paris Commune, the Frente Popular in 1936 Spain was a democratically elected government. We may add 1946 Chechoslovakia (before the 1948 Communist coup it had a democratic elected coalition government with communists in it), or even Italy between 1946 and 1948 (when it was ruled by a coalition of antifascist parties, communist included). Surely, these examples go back to when the Iron curtain was still "being built".
That said, it's true that in the cold war the presence of communists in a government west of the iron curtain was seen as a threat -- and the Italian government, eager to please the newly 'acquired' USA ally, pushed to get rid of such a leftist government (and in 1957 intervened pro-centrist in a political crisis in the small republic).
So, technically the statement on which the question is based is inaccurate. But the first left government with communists into which was elected west of the iron curtain surely scared the West, even if it was in a tiny republic.
It must be said, however, that the government we are speaking of was formed before the cold war divided Europe, ostracizing communist parties in the West. San Marino was exceptional because the communist participation lasted so long.