If by "building" you mean "starbase" then yes.

You just have to build using a "Cargo" size hull, and then save it as a Starbase.

Then, when customizing your civilization's ship and fleet style, you simply put whatever model you built in the Shipyard and various Starbases.

That, I do not know, sorry.

In a customized faction, you can only choose the planet system (Alterian, Arb, Iconian, etc...), and not the way the planet are designed (layout, hexagons grouping, bonuses, mines/buildings). It is done randomly by the game.

But you can change the leader's logo, foreground and background images, the citizen images, the texts when discussing with this civ, the types of ships, the various colours. But Not the planet.

The building are determined by the game, according to the Ben/Prag/Mal ideolgies, and some special traits.

Unfortunately, even in a customized map, you cannot do much on a planet, using the available tools:

You can:

- change its name/all system names

- decide of its type (sun)

- change its basic size

- add any improvements in it, mines, special buildings (from ideologies/traits)

You can NOT:

- choose how your hexs are grouped, or where.

- choose what bonuses (ex: +3 All Constructions) are generated for an hex.

- which hex receives a bonus, or not

Now...

If you really want to go deeper, and play with XMLs, here is the guide:

https://forums.galciv3.com/478190/guide-how-to-create-a-custom-star-system-for-custom-factions

To get unique Improvements you need to link them to an existing Ability or create a new Ability and grant that to your custom Race.

You would add the Ability as a PreReq for the Improvement.