The key question is probably how much extra info these supplements give compared to the 8 CT Aliens Books.
Can't judge Solomani and Aslan as can't afford it, but to give quick summary of the others:
1. MEGATRAVELLER VILANI AND VARGR FROM DGP
(very rare, unlikely to ever be reissued in any form and expensive)
Vilani and Vargr obviously contains a lot of information (but no chargen) for Vilani which is available nowhere else.
There's not really that much more for the Vargr compared to CT though.
Vargr Chargen is very similar with just a few extra Megatraveller skills.
2. TNE ALIENS OF THE RIM BOOK FROM GDW
(also rare but currently downloadable for just $7 from drivethrurpg.com)
Hivers and Ithklur is a love it or hate it book - there is a lot of excellent material not reproduced anywhere else - above all on the Ithklur who are my favourite of all the Traveller races - but the style can be very infuriating and the chargen info does not easily translate back into CT.
Wouldn't pay a shedload of money for a hardcopy but well worth $7 for the download.
3.GURPS ALIENS BOOKS
(AFAIK all kind of in print or at least findable at reasonable price)
Would recommend all the GURPS Aliens books (and Rim of Fire which contains a lot of info on the Solomani and Humaniti which has all the main human sub-races).
As regards the 8 races for which there are CT books, GURPS don't necessarily add a great deal, but the background material is SO much better presented and some effort has been made to ensure that each book has one or two extra races not covered extensively elsewhere (for instance the Ztachk and Gurvin in the Hivers book).
However while a lot of effort was made to keep the main races in line with CT and MT canon, the new GURPS races are of very variable quality - Drakarans are just another race of upright dinosaur-men (of which Traveller already had several) Valkyrie are classic parasitic Bodysnatchers and the Tezcat could have been interesting but have been placed in a completely impossible state in the Great Rift.
(as far as I can tell the Tezcat are the only GURPS Aliens which actually involve significant changes to canon)
4. JTAS collections from FFE
If you don't want the GURPS books, it is worth noting that the three Journal of the Travellers Aid Society books from FFE contain at least a dozen or so alien race write-ups including Newts, Virushi, Ael Yael etc - most of which contain at least some guidelines on CT chargen (and are thus more instantly usable than the GURPS material).
They also have some nice additional background material on the major races (K'Kree philosophy that sort of thing) which expands on the CT Aliens Books.
5. T4 ALIEN ARCHIVES book (not that rare and again available very cheap from drivethrurpg.com).
The T4 Aliens book is indeed pretty poor and contains nothing on the canonical races but does include a number of new races that appear nowhere else (possibly for good reason).
http://members.ozemail.com.au/~iandl57/Page26.html gives you an idea of the contents.