Also, I'm adding another race (not one of the galactic superpowers tho):
Jenelenjei
The Jenelenjei consist of an amorphous organic mass controlled be a central nucleus, able to mold themselves into all sorts of shapes. They once (around 3,000 years ago) expanded rapidly and aggressively, as due to their ability to adapt themselves and assimilate/digest an astounding variety of biological materials, they had less need to develop life support and terraforming techniques. They devastated entire biospheres to feed their growing empire, which, though vast, is utterly dwarfed in comparison to the superpowers of the modern galaxy. However, in the midst of their expansion, the First suddenly began massive excursions into their territory, decimating their empire. The fledgeling Crimson swooped in shortly after, swiftly finishing off remaining centers of organization and claiming the most of the former territory for themselves. Today, most of the species lives as subjects to the Crimson empire, though some survivors of the downfall escaped far away to found a new sovereign nation deep within Arm-Verde. However, this territory is comparatively tiny, consisting of only 231 star systems.
Individual Jenelenjei range in size and mass from similar to a medium-sized dog to around that of a small car. Their size is constrained by the ability of their nucleus to effectively send signals to their gelatinous biomass. While their base state is totally amorphous, they are able to form and harden more permanent structures, such as skeletons and pseudo-organs, and shape themselves with almost total freedom, often choosing to mimic the shapes of other life forms. These self-formed structures can be dissolved and remade as needed, as only the nucleus truly matters. Their forms often look glassy (like a jar-jellyfish), as opacity and color take a lot of skill and practice. Those with highly developed psionic potential can often be identified by their glowing nucleii, and strands of light snaking and crackling through their bodies. Their species practice a form of sexual reproduction, in which two or more individuals will undo their chemical barriers and merge their bodies into one, followed by their nuclei. The bonded nuclei will then intermix and fission into a handful of new, smaller nuclei who possess not only recombined genes but also recombined fragments of their parents' memories.
The modern Jenelenjei are considered to have more tempered and considerate morality than their empire-building forebears, though in their societies there are often strains of resentment towards larger entities, particularly the Crimsons. With some training, a Jenelenjei is able to rapidly destroy almost any biomass they can bring their own into contact with, but there exists an agent commonly known as "J-cide" which rapidly dissolves their bodies and leaves their nucleus totally helpless.