"Obsolete" techs and Stolen techs

Only the tech the bonus is attached to will grant the bonus. Buying an advanced colonization tech will get you the 50% usage it normally grants, but you still need to research or buy the basic tech to get full use of those planets.

Usually the earlier tech is worth researching only if it has a passive bonus or another tech it leads to (ex you steal beam weapon theory without having space weapons, you would need to research space weapons to get the other weapon trees). Of course, this can occasionally lead to hilarious AI exploits - gift an AI Impulse II engines and they generally won't go back to research Impulse I to get the passive bonus. That may have been changed since TA 2.0, I haven't tried it recently.

AFAIK the only requirements are 1) you invade, not spore or flip; 2) the tech is stealable (many are not); 3) they have the tech, and you do not. However, don't expect to steal a tech each time you invade, even if you meet all the requirements to do so. There is a random chance to do so each time you invade. I've never really tested it, but it seems your odds to steal go up the farther behind you are in tech, meaning stealing one tech out of 20 possible is more likely than stealing the only tech available.

In general, which techs are non-stealable?

Quite a few of the civ-specific techs are non-stealable. The Korath's spore technology, Yor charging stalks, Thalan manufacturing/research/starbase technologies, etc. Some techs are simply so powerful the game would get out of hand if you could steal them.

But I can steal the most powerful weaponry from enemies.  I think that really turns the tables on the enemy, especially the dreadlords.

it's a nice piece of software which will allow you to browse through technologies

on the other hand for TA most very specific techs are non-stealable like the yor farms for example

which makes sense since while most races use food, the yor don't really use food being machines ...

That's what I thought also, that civ-specific techs were unstealable (as well as untradeable), but apparently they are. In my last game of ToA 2.02 (i believe that is latest version), I stole Merchant Trade Complexes from the Iconians (which are pretty powerful) after I conquered one of their planets. I also stole Interstellar Bartering and Merchant Emporiums. And I stole Kinetic Streams from Drengin (i think, that may not be unique tech tho, since it always shows in trade screens).

I remember stealing a Torian tech too same game, which I thought was a unique, but can't remember what name was. It gave a 20% overall research bonus and could be built one per planet.

Quite a few does not in any way imply all. Offhand, I'd guess maybe half are stealable, depending on which race you're talking about.