There are several communist parties in Greece each tending to follow a different doctrine of communism, Maoist, Marxist, Trotskyite etc. Syriza formed as an umbrella group of people who had left these various factions. As such it was far left. It became a party, I think for the 2012 election, because the way they would be dealt with in an election as an umbrella group is different from the way a formal party is treated. As a party it attracted people disaffected with other parties, mostly from PASOK and the Democratic Left. KKE, the biggest communist party and the one that got seats in parliament, hate Syriza because they see it as having stolen their ground in the political arena. They are seen as traitors to the communist cause so there is little chance of any alliance there, ever.
Because of their roots they were initially very anti-Euro and anti-EU. That view has softened over time possibly because of entrants from other parties but probably principally because it is clear that the vast majority of Greeks, typically around 75 %, are pro-Euro and pro EU. People voted for them because they promised to end austerity and Greeks are fed up with austerity, for very good reasons.
Syriza has a very accepting policy when it comes to immigrants. For example they want to grant nationality to a range of immigrants, such as children of immigrants born in Greece. I don't see Syriza as a direct threat to ex-pats, therefore. They may represent an indirect threat in that failed negotiations on debt could result in both a withdrawal from the Euro and a withdrawal from the EU. I don't think that extreme outcome is likely but we have discussed previously the possible problems that a UK EU exit could cause and I guess those problems would be the same for a Greek EU exit.
Independent Greeks, on the other hand, are almost as right wing as Golden Dawn. They are racist, xenophobic, homophobic, anti-Semitic and their policies reflect these views. I cannot see them getting any of these policies adopted so I don't think they represent a threat either. Even if they did start to pursue these policies, initially at least, they would be concentrated on illegal immigrants, Jews, Muslims etc. WASPS would have little to fear.