Which Rush? Don't know. Although I don't think the Raskvel have joined the U.G.C. Nor have the Sydians (but that might be more due to how their touch corrodes technology). The Gabilani are in negotiations to join the U.G.C. and one of the current provisos is that they stop enslaving the Raskvel.
It might be a case that the U.G.C. has certain terms or criteria to be fulfilled before allowing another member race to join. If we take the real world example of the Copenhagen Criteria for joining the EU as perhaps a guide:
Membership requires that candidate country has achieved stability of institutions guaranteeing democracy, the rule of law, human rights, respect for and protection of minorities, the existence of a functioning market economy as well as the capacity to cope with competitive pressure and market forces within the Union. Membership presupposes the candidate's ability to take on the obligations of membership including adherence to the aims of political, economic and monetary union.
Something similar is seen with the Federation in the Star Trek universe, where a petitioning planet has to have a certain level of technology (not just getting out into space, but being able to travel beyond their home system via warp travel), as well as having a stable government, values of benevolence, rule of law and so on.
As for half-Raskvel, I can't be sure, but I think the Raskvel might just produce full Raskvel when the eggs hatch.