"Also it is not unimportant that extreme forms of socialism have central places in Catholic life--the first Christian community as described in Acts, and all religious orders with vows of poverty."
Absolutely. Those particular voluntary "socialist" environments seem to work well. Leo's, (brother (were he real), not pope) order would be a good example.  The Mayflower Compact's similar voluntary socialism failed in spectacular fashion, however.  Voluntary is the key concept, to me, and I don't believe that Leo's concept of "socialism", as discussed in Rerum, included the voluntary but, rather, a socialism that was forced (and enforced) by the state.  
That there "is no modern capitalist economy that does not have socialistic elements deeply embedded", I think, is quite true.  Even the old "strictly socialist" economies had side (dark) economies running that were far more capitalistic than they would have admitted at the time.