Actually, it's much better to keep that one guy that you can easily protect around all game and buy everything from him and send all your trade routes to him too, then build a ton of eco-trade boosting starbase on the way and you're set for life (Don't worry, he'll use that money to buy more tech from the other players, which you can then buy off of him for dirt cheap - I like to do this instead of whoring tech). Unless your galaxy is so small and planets are so few that a PQ 15 planet would change everything, it would be much better to just start killing other people first.

In my experience, tech trading with minor races is a great strategy in the short game, but in the long haul I find I out-pace them in research to such an extent that they become useless as trading partners. For my purposes, I like to keep them around long enough for them to build at least two capitals before I annex them; you can only build one of each capital yourself, but there is no rule against "liberating" multiple caps from other races.