Defensive orbitals yes. Things like a shipyard no. Although you will require a shipyard, or what not at any planet you want to build larger ships at. Same goes for things like orbital refineries. So most orbitals, no, unless they are offensive. You can build more than one, but there isn't much point.
You own all the planets, but only one has a colony. A colony ships adds 1 million population to a planet. Depending on your tech levels will be how fast that population grows. It can be VERY fast at higher levels. The planets in your system start as "claimed". We did this so if you have someone starting close to you and they find your home world sector we didn't want them to be able to grunt rush you by claiming the planets in your sector.
Here is the skinny on planets, unclaimed, vs claimed, vs colonized...
Unclaimed:
The planet is unowned and no basic mining colony is set up. The planet will not gain resources since nothing has been placed.
Claiming:
Claimed planets will gain resources that can be harvested by the owning player. Imagine if you will that claiming a planet places a mining colony at the planet. While the colony doesn't grow, it will start to gather resources. The player can then use harvesters to acquire the labor/production units from the planet, and bring it to their main planet. It will have base defenses so that if another player came in with a harvester and attacked the planet to claim it their harvester would most likely be destroyed. Although it doesn't take much to take a "claimed" planet.
Colonized:
The player has claimed the planet, and has placed a seed colony on the planet. You can still use the planet as a mining colony and harvest resources, but you will also have an active growing population. This will allow for factories to be built on the surface of the planet to start building structures. This is a much harder planet to take, and you aren't going to use a little cruiser to acquire the planet.
Harvesting Planets:
You can set up a route on a harvester and have it ferry resources from one location to another. When you set up a route the harvester will automatically continue. If it reaches the source planet and there are not resources to be taken it will patiently wait until it has gathered enough resources to take to the destination planet. You are only able to harvest from a claimed planet, or a colonized planet. Unclaimed planets will not yield anything. It will just cause your harvester to fly there.. see there is nothing, then return home.
Yes. With a maximum of 9 factories.