I just finished up a run with Silicoids, and I have some notes (mostly confirming things already stated in this thread, with one disagreement with my prior posts).
Silicoids can't get the techs that reduce industrial waste (ex. Reduced Indus Waste 80%), that clean up pollution per bc (ex. Enhanced Eco Restoration), or that allow settlement of hostile worlds (ex. Controlled Barren). I never saw them for trade and I never stole them in planetary conquest. It's mostly minor, but that's some miniaturization and production bonus that apparently isn't available to them.
I really don't understand what the heck is going on with Silicoids and waste. Waste levels were very erratic throughout the run; at the end of the game, only some of my recent conquests had waste on them, while every other planent had no waste and no eco spending (recent or otherwise).
I do know that waste doesn't lower max pop on their planents, but that's about all I can confirm.
They can get Improved Terraforming +x to increase planetary population. They can also get Cloning, which is a HUGE boost for them; Cloning massively reduces the heartache of trying to get new colonists. At that point, they can almost match other races that have cloning in population growth numbers, but they have to spend basically the entire planent's production to do so (other races only end up spending half). Without cloning, they were massively behind in population growth numbers, even with maxed ECO spending. Planetology research was mostly just researching Impr Terra +x over and over again.
I didn't see Soil Enrichment or Atmospheric Terraforming come up for research, but that could just as easily have been bad luck with the RNG for techavailability. Only the Klackons ever spread beyond their homeworld, and they never researched either one either.
I've confirmed that even with some idea of what I'm doing, I still hate the Silicoids. They're pathetic at tech research, and their pop growth penalty really hurts both colonization and invasion. I was really hoping to see the bioweapon tech so I can virus bomb enemy populations, but I wasn't graced with it so I had to invade the really slow and tedious way in order to snatch techs via conquest.
I feel like they really need either not to be screwed on tech research (they don't even have Excellent in Computers, merely Good, and everything else is Bad), or they need the pop growth penalty removed. I think giving them Excellent Computers and two or three other categories as Average would be the stronger boost, but maybe if they didn't suck at populating all planents they'd be more viable. Or at least they wouldn't feel so bad to play.
That galaxy was great for Silicoids, and my starting position was also quite good compared to all AIs (only Klackons also had a decent start), and it still felt like a slog early, and tedious until after I picked up Cloning.