That the echinoderms show a fundamental unity in design doesn’t really have to do with picking Hallucigenia as a starting point - and in fact that specific kind can’t be for all of them, since there are already others from the Burgess Shale era, like Gogia, which aren’t particularly similar to it.
Suggesting that Hallucigenia might belong with them is an interesting idea, but one which seems to be mostly justified by how ambiguous the squished fossils are. Something that is definitely less ambiguous is Orstenotubulus, for which there is some degree of a three-dimensional reconstruction (pdf). This fits in very well with the lobopods, other animals with paired stubby legs represented by modern onychophores, and I think Hallucigenia seems closer to it than it does to any of the older echinoderms.
