Reaserching specific terran or desert will give you an ability to increase the max population you can have on those planets, the bigger your pop the more tax income you recieve.
Also as Mathew the Mage mentioned you buy/build infrastructure using the planet upgrade button located on the bottom right hand side of the heads up display, once you have clicked on a planet. Its the green one with longnitudinal/latitudinal lines on it. After clicking this, click the button wuth the half man( must turn green first). This button upgrades your infrastructure, also allowing your planets pop. to increase thus more income. Note; if you upgrade too many of these straight away it can cost you for awhile. Oh! your home planet usually comes with these upgrades at the start of the game and does'nt need doing(at least to begin with), all planets you colinize will have none.
In answer to your last qestion, you don't have to put a starbace in a spectial system to continue a chain. It contiues through systems where you can't put a trade port (including between stars). You just have to avoid your opponant owning a planet/asteroid in your chain, this will brake it. Putting a star base with a trade upgrade in these spots will improve your trade income though.
Hope this helps. I"m sure you will work it out 
