M.D.
I'm not quite sure what you are asking, but I have a lot of experience with Thomas Trains, LOL. The little magnetic ones hook together as solidly as a half-inch round magnets can. You can hook 3-4 cars together and if he pulls slowly, they will stay together. If he jerks them or pulls very fast, even just 2 cars will come apart, just like jerking a magnet off of the fridge.
I don't think there is a right or wrong set to buy with the Thomas stuff. We mostly did the Trackmaster plastic track and trains that go with it (not the mini-ones) simply because that is the 1st set someone gave my son, and we wanted everything after that to be compatible. A friend of mine had the wooden sets for her son for the same reason - that is what they started with, and they wanted to be consistent and not have mixed sets that are incompatible with each other.
We had two small mini-sets like you describe and those were nice because we could take them with us when he wanted to bring a toy along on a visit to a relative or friend's house. But mostly we had the plastic Trackmaster sets to play with at home.