There's a tab at the top of the buildings list that allows you to change between newest buldings, older buildings and all buildings. You're still allowed to build older buildings in Twilight, although the maintenance costs revision does mean that some newer buildings can be more cost-effective.

At a certain point you end up effectively paying lots of money just to keep the extra citizens happy (or if not happy then at least content enough so they don't revolt) so I think the best idea is to work with whatever population will be cost-effective on a given planet.

Planets with high population do tend to have better influence so with some cultural improvements they can generate quite a bit of tourism income. However if you're playing a race without many influence options on the tech tree, tourism is always going to be a small slice of your income, unless you get the tourism boosting mega-event.

If colony maintenance is a problem then you need to raze factories or labs that you don't need. You can always build them up again when your economic crisis has passed. More importantly, if you're having to run with the production slider over to the left because of a lack of credits then all those extra factories or labs are keeping you from using your planets cost-effectively.

So is a far less aggressive colonial rush strategy better in ToA? Even if an aggressive colonial rush strategy is expensive early on, does it not pay off long term?

Just like manufacturing, research takes longer when your production slider is over to the left. The proportion of your spending as determined by the military, social and research sliders also haa an effect. The initial colony tile provides some research at each of your worlds but if there isn't much funding the research takes a while. When you add dedicated research facilities on a planet, there's more research that can be funded through the sliders.

Economic starbases can also give a bonus to your research and manufacturing, and you only pay half as many credits to finance the extra as you would for the base research points.

As for early colonial rushes, it can be good to establish quite a few colonies early on because in the beginning you don't have tile-effective ways to improve morale; so having places to ferry your homeworld citizens to is a nice thing. Long before you ever get troop transports, you can use colony ships to move citizens between your planets. I wouldn't rush to grab every old world though, as every colony costs maintenance right off the bat. The ones with asteroids close by make for good choices because once your space miner has set up mining outposts, those colonies that are seeing the benefits of mining operations have more industry points, and that's great for cranking out the next wave of colony ships. More to the point, you don't pay maintenance on great big rocks like you do for factories.

When you need to spend some of your hard-won tax money on research, those worlds that aren't near to asteroids and aren't likely to be churning out lots of ships make good colonisation targets. Most importantly, planet quality determines how many tiles are initially useable. So far I've never seen any bonus tiles (improvements to morale, research, manufacturing etc) on tiles that need improvement to become useable, so planet quality should be a rough guide to which worlds you want to acquire first.

That said, some low-quality worlds actually end up with a pretty decent PQ once they've received some tender loving terraforming, so whether you rush for them depends on just how long-term your plans are.

On the whole, a rush for colonies can be worth it later on because of the tiles you will have available to generate income and do research and manufacturing, but only if you can hold onto them and only if you can eventually run your economy with the production slider over to the right.

I think the main reason for High Maintenance is Metaverse. IE back in the day get players with Million scores per game cause of high military score. Now its harder to get those nice high scores. But it does make things more interesting.