Okay, I see what you're getting at, and it's a good point; but as a minor quibble, "starfish aliens" are, to my reading, pretty completely alien, while rubber-foreheads have strong similarity. You could have species 1, 2, 3, 4, 5... with each neighbouring pair being rubber-foreheads relative to each other, and becoming less and less similar as you travel down the line, but given those constraints I don't think you can have proper starfish until you're a good distance along that line; say, 10+ spaces. (Starfish and rubber-foreheads are extremes of, respectively, "different" and "similar" - and there are a lot of gradations between those extremes).

Of course, in any realistic lineup, it won't be a neatly spaced line; number 4 might be missing entirely, and numbers 5 and 6 surprisingly close, and so on.