Bowser and a Buzzy Beetle are definitely not the same species. Same family at most... then there'd probably be a split for bipeds and quadrupeds, and THEN you start getting into the specifics.
The idea I was meaning to get across is that the Koopalings, Boom Boom and Pom Pom aren't the same as Bowser and Jr., which comes from Jr.'s horns. None of the Koopalings, nor Boom Boom, have them, despite (most of them) clearly being older than Jr. In fact, now that it's not the case that they are "Bowser's kids," that could also mean they aren't all "kids" either; possibly all full-grown adults, with the probable exceptions of Larry and Lemmy. Although, Boom & Pom don't have spikes on their shells while the 'lings do. So maybe not. Or maybe so, because the shells are removable and interchangeable. Or maybe all mentioned are the same species and are simply manifesting very different genes, like different dog breeds do. These are the things that fascinate me
Dealing with a retcon like this is interesting. It changes so much and so little at the same time!