You COULD drop every d20 3.5 OGL species in. I use the original Samardan Press version of Cepheus Engine to convert to Animals specifically because it uses Intelligence and an analog for Education (instinct) which d20 Wisdom can used.
I have broken down conversions into 4 types types: Terrain Events, Animals, Sophonts, and Constructs(Robots).
Animals, Magical Beasts and Vermin are Animals
Constructs are effectively Robots
Dragons, Giants, Humanoids, Monstrous Humanoids are Sophonts.
Plants and Oozes become Terrain Events unless can make attack rolls OR have an d20 Int greater than 0. If their d20 Int is 3+, they are Sophonts (ex. Treants), otherwise they are Animals
Fey and Outsiders are Animals or Sophonts depending on d20 Intelligence (null,0-2=Animals, 3+=Sophonts)
Elementals and Undead are not subject to stun and critical hits, so they are like Robots with Hull and Structure OR Animals or Sophonts depending on d20 Intelligence (null,0-2=Animals, 3+=Sophonts)
Aberrations typically Animals or Sophonts depending on d20 Intelligence (null,0-2=Animals, 3+=Sophonts) , but being so unnatural might be Constructs or even just a Terrain Feature
Deciding on Type right now is important as things like stats/skills/natural weapons/Alien Traits availability is determined now, and in play determines how psionics/spells influence it (Is it a sophont or not) is it an animal or sophont
Once you decide on the Generation Type (Terrain Events, Animals, Sophonts, Constructs(Robots)) you can generate some systematic things.
First strip them of all equipment, even if the description says something like "they are always encountered with (equipment)"
Stats are typically multiplied by 2/3 to convert to their Traveller stats.
STR, DEX, CON become STR, DEX, END
CHA translates to CHA (if your version of Cepheus uses it) unless you are a Traveller Animal in which case your Number Appearing is your Pack Score implies your Pack score. If you are using only SOC, derived CHA value's bonus from (-2 to +3 for humans) may imply some natural Alien Trait bonus or penalty to reactions. (Those elves and fey can be sooo beautiful at times).
Intelligence and Wisdom translate differently
If a sophont or robot d20INT = TravEDU, d20WIS = TravINT (yes that is correct, swap them!) This allows you fit many otherwise "dumb hyumanoids" to have roughly equivalent and write it off with the "Feral" alien trait. Especially if you are creating a race template since a standard race human rolls 2d6, you cannot vary Traveller Intelligence too much on the low end (2d6-1)
unless you are going to create a "stupid" Alien Trait or some such.
If an animal, d20INT = TravINT, d20WIS = TravINS (Instinct) . Dividing by 1.5 leaves (former) Animals and Vermin with a Trav Intelligence of 0 or 1 in all cases. (Former) Magical Beasts, being sort of animal/sophonts after dividing d20Int by 1.5, subtract 1 from the result then divide that remainder by 2. Finally throw the 1 back in. ((((d20Int / 1.5) -1 ) / 2 )+1) = TravInt . d20 Wisdom becomes Instinct.
This can lead to a non-sophont's having Int higher to a 1, which is Stellagama discourages, but does not automatically disregard.