That depends on how you choose your cathegories. You could also say there is the big cathegory A of very humanlike aliens, cathegory B of aliens that primarly resemble a specific earth animal and than there is cathegory C: other. My point is that categories are made up by choosing criteria you consider significant.
With a good category things grouped together feel like they are a group. For a lot of Sci-Fi humanoids are a good catagory since they share a lot of characteristics. On the other hand something like B from above wouldn't feel like a group, for example: a tiger an eagle a squid and a wasp, do they really feel like their one group?
If we have humanoids ingame they should be no more common they any other type (that probably means no more than two I haven't counted though). Half humanoid, one of A, one of B... one of M. wouldn't work too well.
It would lack any feeling of overall cohesion, having a bunch of very similar humanoid aliens in a galaxy where every non-humanoid was highly unique would give a similar sort of feeling as having two seprate design teams that weren't talking to each other. I'm not very good at putting feeling about things like this into words, sorry.
Secondly, if half the alien races (say 9) are all humanoid then it would be very hard to have 9 creative and innovative outward appearances if your limited to the same body shape. 9 different rubber foreheads or animal faces isn't the hight of creativity and hasn't been since the ancient Egyptians did it.
Finally you'd have to justify why no individual body shape save humans got more than 2 races (if that), but for some reason 9 separate races evolved into the same body shape. Normally we don't put too much focus on realism, but this is the backstory rather than the gameplay.
But, when building a race, one has two options, to go with the 'natural' behavior for the form, or against the 'natural behavior'.
Forms have a lot less natural behaviour than you might expect. Dog: a friendly loyal companion, eats food provided by its owner. Wolf, same shape, a vicious predator.
Its easy to see why people think crystals are cold and analytical, all those hard edges and angles sure look it, but when you think about it there is no logical reason for them to be so.
And its not as clear as with the form or against it, you could mix the two: crystals who are intelligent, analytical, logical but care about other life. Or you can go on a tangent, replace analytical with philosophical. Or of course you can do what people here seem to be quite good at and create very unique forms that don't have too much of a history.
And if you go against the 'natural' why? if it's just to upset the 'normal'
That one of the reasons I do it
keep in mind that at best you might hope for is to succeed and make people uncomfortable with the race. . . . not normally what one goes for when creating a race? no?
Do you have any examples of that actually happening?