One of the first things that you will learn about LFS' is that most LFS' want your money. They will tell you just about anything to get you to spend more money (and this is coming from someone who has worked at several LFS'). When I worked at the LFS' I did, I would pride myself in giving 100% truth to my customers, regardless of whether it wasn't making the store any money at the moment. My thinking is that to get someone to spend $200 in one visit and then never to return is not a good thing ever. However, if you are honest and helpful, they may only spend $20, but they will be back 10 more times because they have found someone they can trust to help them. These are the type of people you need to find. The ones who know what they are doing, and looking out for YOUR best interests, not the LFS. The more and more you are in this hobby, the better eye you will get for that, trust me. Take what they say with a grain of salt and always get a second opinion until you know you can absolutely trust what the person is telling you.
I would just stay with the tank you have. It is a fine size.
Fresh seafood would be great to feed your fish. The most important thing you need to know is variety. If you and I ate pizza every day, we would start to dislike it and get very unhealthy. Fish are the same way. They need nutrition and variety. Tetra Marine granules are okay, but only in a large diet. It is not okay if it is the only thing being fed.
I would pick up Salifert test kits for pH, kH/alkalinity, and nitrates. Salifert kits are the most reliable on the market. You can use the cheap ones (Aquarium Pharmaceuticals) for ammonia and nitrite, since no established tanks really get either of these things. The cheap test kits for these will notify you if there is a problem. You don't need precision for these, because, any sort of ammonia or nitrite in your tank is bad.
