I see what your saying. The moment a player makes changes to a carefully balanced planet that benefited in the past from another tech then all bets are off! Let the population nose dive sure. But I think it not wholly unreasonable to at least let the new owner prepare to catch the sudden downturn by not immediately introducing it on conquest but instead delaying it till they start disrupting the planets status quo.
As for cross pollinating your Tech trees. Not generally in favour of it either. Maybe later if you capture a ship you can keep using any specials it has but you cant repair it so eventually you’ll loose that specialist resource. I could just about go for that but not a lot more.
If we could all buy Synt Hydro stations then mid to late game we would for the added population!
No to cure that, the mechanics of Hydro Balance and Temp as they impact Racial growth need at least one more balancing pass. It may be very “Meta” but its a terrible shame that getting a Synt player to Hydro adjust your fave planets to 90% is actually a desirable thing. But that’s really another thread perhaps…