Regarding shellfish, I think you have to use some judgement too. I eat shellfish very, very, VERY rarely, lol ... I love it but it's such a rare delicacy. I'm still waiting on my "birthday dinner" (over 10 months past due lol) because I want fried crab claws. I love crab and lobster. But even assuming I could afford it, it would be completely against the spirit of the fast to have crab and lobster every fasting night.
I didn't even know for the first couple of years of being Orthodox that it was allowed. I think once there was a feast/fast day and we had a parish dinner, and they served pasta with sauce and a few shrimp. I was confused lol.
All of this and any questions that come up because of it are best discussed with your priest. He can better explain any misconceptions we get.