Remember that playable faction must be more or less humanoid. Their heroes must use the same equipement that other heroes uses and their secret tech units also can't be totally different than those of other factions.
NPC factions is where a really weird aliens work as they don't need to be humanoid at all.
While i agree i would like to say that the reasons you give for the playable factions to be humanoid in design is one more reason for making a race that break that rule. Since all the playable factions so far has been humanoid, sound to me, as a good reason to see if there can be make a playable faction that isn't. People have been asked for truly alien things, so why not something truly alien. Think the voidbringers but playable