There are six major races in the Official Traveller Universe. All are very well described in a number of books in different Traveller rules versions. This is a good source of information to help you with the races for your own Alternate Traveller Universe.
The problem with the major races in the OTU is that they all are placed in the OTU and a few of them may be difficult to re-use your own ATU. Described below are my opinions (in alphabetic order) why the races may be good or problematic for re-use in an ATU.
There are lots of reasons for creating your own ATU. One reason may be that you are bored with the OTU, another that you don’t like all the conflicting canon and another reason may be that you just want to create your own universe.
Aslan are a race of catlike aliens. Even though that might seem like a sci-fi cliché, the Aslans are well described and would probably work well in an ATU with only some minor changes.
The Aslans have been described at about the same time as the Chanur novels was written. It is difficult to guess what cross contamination that has occurred. My guess is that it was in both ways.
Spoiler warning! Mark the hidden text below to make it visible.
The Droyne is one of the most important races in the OTU. They (as the Ancients) transplanted humaniti to many worlds in charted space. If you don’t want this in your ATU, then the Droyne cannot be the ancients.
The Droyne is well described and would work well in an ATU as an old mysterious race. You may want to ignore (or change) the secret part that is hidden above.
The Hivers are the starfish aliens that actually are starfish. They are also good at manipulating other races to do what they want. They are also very good at mathematics and use a base-6 system. Their computers and robotics are very advanced. The Hivers don’t like ground combat and use other races to help them with that.
The Hivers could work well as a strange race in an ATU setting.
There are the Solomani, the Vilani and the Zhodani and lots of minor human races. In the OTU all human races (except the Solomani) has been taken from Earth a long time ago and they have then evolved independently. This may seem like a stupid idea to re-use in an ATU unless you are a trekkie or likes von Däniken.
Humans from Terra (Solomani) can be borrowed from the OTU. But these are not the good guys, so you may want to change them.
Other human races will be more difficult to re-use in an ATU.
These are horsemen or centaurs. In the early versions of Traveller, they were actually called Centaurs. I think they can be a good foe in an ATU with their vegetarianist agenda.
Big strange races like this (with strange motivations) usually work best as NPCs.
In the OTU these dog people are uplifted Terran dogs. Although Vargrs are one of the best described races in Traveller, they belong in the OTU and cannot really fit into an ATU as they are described.
If they are uplifted in another way than in the OTU or if they are not actually dogs (just like the Aslan are not cats) then they could work.
Works well: Aslan, Hiver, Solomani, K’kree
Problematic: Droyne, Vilani, Zhodani, Vargr