Hi!
When should the colonization techs be researched?
IMO immediately after you get all level-1 buildings (xeno lab, factory, advanced market) and other early and cheap techs that give bonuses to whole empire (planetary improvements, xeno medicine(?) for 10% bonus in growth...).
In my current maso game I've waited too long with these planetary techs, and neighboring AIs have beaten me to ALL of close planets, as each one researched tech for a different environment. Fortunantelly I had a quite old savegame from previous night, so I'm now a happy owner of 2 big radiated planets and 3 nice toxic ones

, and with double the amount of usable tiles. Still Altarians beated me to one medium toxic.
In future sandbox DA games I'll not be waiting so long anymore.
BR, Iztok