Hi there,
In Thassos, or Greece in general, we had similar experiences.
When some greek friends take us to a greek restaurant, they order food for the whole table, like some fried cheese apetizers, dips,... when you are for ex with party of 4 they make shure, you have at least 4 pcs of each.
as 'main course' they put a large plate with the grilled meat, biftekis, souvlaki, .... whatever you want.
There is a completely different way of eating, it's like a family get together and everyone takes whatever he wants, same story for the drinks. At the end there i allways too much, and you are filled like a balloon.
Most restaurants don't make a problem of it, in case of 2 persons, you can choose for exemple 3 or 4 starters on the table, there is more variety of tastes and you are not frustrated of sending back full plates...
I think that what you said from Skiathos is also true for Thassos, thats also our experience in our 2nd hometown Potos. and we love this way of eating...
So, feel free to ask whatever you like in the taverna of your choice, if they consider it as rude, go elsewhere the next evening

.
When I was the first time on theisland back in 1988 when I was a little boy I remember we missed the traditional food from home (after 3weeks) and my mother was allowed to go inside the kitchen from the taverna in Theologos to prepare a meal like we used to eat back in Belgium (think was cooked potatoes, with steak or sth....), this shows how friendly and flexible most of the greeks are...
Hope you enjoy your holiday!!
Bert