I've read a handful of Stalin biographies, but none mention meeting Trotsky in this period. While Trotsky was interested in psychoanalysis, I've never heard of him meeting Freud directly.
/u/commiespaceinvader answered a very similar question here only a few weeks ago actually. In line with what /u/21stCenturyMachiavel has said in this thread, /u/commiespaceinvader says that Trotsky and Stalin did meet. According to /u/commiespaceinvader it is within the realm of possibility that Trotsky and Stalin were at the same place at the same time as Hitler at a cafe they all frequented, though they never met. Tito, Freud and the Archduke Franz Ferdinand were in other parts of the city and almost certainly had no interaction with any of the others.
Slightly unrelated, but why did Vienna seem to host so many people who'd go on to become 'extremist' political leaders (those above, plus Tito iirc). Is there any reason for them all to want to live there, or is it simply coincidence?