I think you misunderstand me then. I'm not talking about the tech tree changing for any race. Just that each race would get a few low-level techs automatically for free at the beginning of the game. A little head start along a research path that would match the background of the race. The player at any point could choose to research anything anyone else does.
I am not misunderstanding you. What you said there in the above quote is what I mean. The player would be able to choose a race pick, which chooses a tech. The player can in this way change what starting race techs the player has. There would not be a tech tree for each race, but one tech tree with a selection of techs linked to race picks.
There's no need to "simulate the effect of different technology materials not mixing well." by shuffling the tech tree around— if the tech tree is well done. Technologies will be based on previous techs in a logical progression.
This is the issue I would like to see. Organic/metal/crystal/energy techs. A true to life example would be that we humans are so used to building 'metal' ships and know nothing about building organic ships, so for us to build an organic ship would take a long time (research wise or construction wise).
If both 'metal' ships and 'organic' ships are level 1 tech, then the player can quite easily have both ship types easily. I want it to be possible to have both types of ships, but for it to be a little bit more difficult. If the humans (metal ships) got their hands on an enemy 'organic' ship, then they can't easily steal tech or reverse engineer it. But they can possible hack and force their way into having the means, only it takes ages to build or the tech is available but takes longer to research.