Megara, I know how it feels when you see feral cats on holiday. I've recently come back from Greece, well, Kefalonia, and I found it heartbreaking seeing all of the starving cats there. There was one in particular I befriended, a sweet calico who I named Meli (Greek for Honey). This may sound harsh but would it be a good idea to bring them back with you? I can understand feeling attatched to them, and I realise they'd have a much better life with you, but for every cat that gets rescued by well-meaning tourists there are hundreds more who aren't so lucky. I can see you obviously care about them a lot. But, as you said, it might be difficult to take them back with you. Also, when I hear stories of people who spend a lot of money in rescuing feral cats from abroad, I can't help thinking that money would have been better spent if it had gone to an organisation designed to help feral animals in Greece. That way, it can be spent on things like neutering which are essential to help stop the feral problem.
I've found the names of some charities:
Greek Animal Rescue
The Greek Cat Welfare Society
Sorry, but the addresses were British ones and I couldn't find any web addresses