I always put one transport module on a cargo hull and add on all the engines it can carry - that lets them move rapidly to the front. When I load them up I always go for the maximum - I think it's 3 billion. I never load up more than one transport per planet until the population recovers - and I try to load transports in systems that have more than one planet to draw from. Invading with two of these fully-loaded transports may be overkill, but it means I can certainly take the planet and have a transport survive. The next turn I can load up the transport and move on to the next planet.
The planet you recruit from will pretty quickly expand its population to the limit - which is a limit for the people actually on the planet. Colonists or troops taken away no longer count toward that limit. Once you invade a planet, the survivors of your invasion troops and the surviving civilians form the population of your new planet.
You cannot have a standing army. If your world is invaded the entire civilian population will fight.
I never bother with defensive buildings on a planet, or with leaving a ship or fleet to garrison a planet. My fleets kill any enemy transports and then lead my transports forward.