If you want a trade good or any improvement to show up when you've researched a particular tech, just look up the relevant tech tree (there are racial tech trees for TotA, which have extra techs in addition to the main TechTree.xml which they refer to) and find the InternalName of the tech. Then enter that as the RequiredTech for that improvement. If you wanted it to be available for a particular race only, you'd need to make sure it's a tech that only they get. Or if it's just to be available from the start, just enter None for RequiredTech.
There isn't a limit to the credits you can have in your treasury, but after building up a surplus of 10000bc your trade income is halved. After building up 20000bc, your economy suffers a slight penalty so your tax income may lessen. There may be greater penalties as you build up a more ridiculously-sized treasury. If you have something to throw the money at, be it rushing improvements or ships or ship upgrades, or even paying off leases early, then there is an incentive to keep spending it. If you're making a pile of money despite these penalties and you want to save up and upgrade all your ships of a particular class, then you can do that too.