I think that's hard. Any "normal" alien unit has to be able to carry and use the alien weaponry, and any model has to fit in the 1x1 box. That limits the amount of alien-ness you can put in.
True that grasping and wielding weapons requires either hands or tentacles, but there's still things you can do with those for some variations.
Fitting aliens into a 1x1x1 box isn't hard, the blood spider is already an example of this. Fitting them in a 1x1x2 can be done using a number of nonhumanoid but upright forms. Like a mantis form, bird/dinosaur form, cobra/hydralisk form, a floater-with-hanging tentacles jellyfish (or zerg overlord) form, etc.
The shagar (or whatever it's called) is almost an example of this, and its tail looks like it even might violate the 1x1 boundary a bit.
Plus, humanoid shapes have the advantage that existing animations can be used for them, and animations are even harder to come by than models.
And, well, we already have UFOPaedia articles and autopsy images for the existing aliens, so we're probably not going to replace them in any event.
That's why I ask this with the assumption that you are provided full content of similar or superior technical quality, for each replaced design- model, textures, animations and research depictions. (The UFOpedia articles would remain the same for every sentence not including words like "humanoid" or "anthropomorphic").
I want to know if you folks would be game for something like this, if content availability/quality was not an issue and the designs fit as well as or better your theme of 'realistic sci-fi-horror'?