Last year in late August we had the choice of the Blue Star Naxos or the Hellenic Seaways Nissos Mykonos. Both departed at around 0730 from Piraeus to Syros. We opted for the Hellenic as we had been on the Blue Star countless times already. The two ships are very similar, so it does not matter much. I believe that there are daily ferries each day around this time (we went on a Saturday).The ships fitted in nicely following an overnight flight to Athens. Check the Blue Star and Hellenic summer schedules when they are released.
This year on Thursday at the end of August there was also the Nissos Mykonos from Hellenic Seaways at 07.35AM and a Blue Star ferry at 07.30 PM. But it will be waiting for the summer schedules to be published to see if it will be the same. Certainly with the fact that Attica Holding (mother company of Blue Star Ferries) has bought over 90% of the shares of Hellenic Seaways and thus will be the owner of both companies. So maybe some lines will be changed to avoid competition between between BSF and HS.Edited: 12 December 2017, 07:08
Apologies, when I said "last year" I meant in the late summer of this year, 2017. I am already thinking about next years trip and appear to be getting ahead of myself!
Syros to Paros will be more of a problem than getting to Syros. There isn't a service that runs every day between the two islands at the same time, but on some days there are early morning services by ferries based on Syros that go to different places every day and there is a late night service on Blue Star sometimes. However, it's always possible to do it by going via Mykonos and changing ferries if you have to.
I have looked at Syros to Paros and Syros to Naxos for the first week of September this year. For Syros to Paros there was at least one ferry every day, but most days only one and the departure times vary between 07:00 and 21:25. For Syros to Naxos there was a ferry every day except Wednesday.