The arcology is a concept by Italian architect Paolo Soleri, from "Arcology: A City in the Image of Man." They are huge buildings, in whatever configuration (skyscraper, a floating city, whatever), with a hundred thousand residents, and nature on the outside, whether beautiful or an uninviting waste.
They are really a pipe-dreams of utopians, who don't understand the nature of man. In that sense, they're doomed to failure. This is Star Wars, where that stuff doesn't matter. By all means, create one or several, and bring them on in. Some certainly already exist, in true or related form, in this vast setting. Byblos, a busy Imperial world in the Colonies, has Starport Tower 214, a tower with 300 levels devoted to starships, 200 of which are not for repair bays. Those 200 levels can hold 115,200 bulk freighters. There are other starport towers on Byblos... and they are dwarfed by some of the other towers on this planet. And Coruscant? That world has a planet-wide city, 2 kilometers deep, swallowing everything. Certainly these sorts of designs and ideas have gone farther out into the galaxy.
This page is for guidelines on making arcologies, whether there's a single one on a world or in space, or for even being a mayor or administrator of one of many on a Core World.
The Free Planets guidelines and rules apply for this too.
Population: Arcologies seem to be designed for 100,000 people, on the pages I've seen about them (I haven't read all that deeply). This is Star Wars. Feel free to go several times past this number, but keep it reasonable. A million people is probably too many.
Income: Incomes are planetary, or even based on the entire star system. The economy, trade, productivity of the people on the particular arcology, etc., are all factors, but it will remain a factor of the planet. Only an arcology in space will generate its own income. These will be lower than what's otherwise acceptable for free planets.
Starport: Some levels of an arcology might be set aside for a starport. If it's PC-owned, and independent of the arcology or planet's government/administration, it can generate income from docking fees and is approved seperately under the starports branch. It's class is a topic for that branch.
Defenses: A floating city could probably have surface-to-space blaster artillery on it, but a tower might be too unwieldy for the larger types. Both directly endanger the structure when placing such an obvious target on them. Regardless, no one is starting with these. Lesser weapons are available, but not too many: the RSDP is over 4 kilometers tall, but only has 80 capital-scale and 40 starfighter-scale weapons, essentially the armament of an ISD I in each fire arc.
Stats: It's better off to have stats for these structures, in addition to the planet's stats. Most or all are capital scale, though the massive Imperial Palace on Coruscant might well achieve Death Star scale. Space-based ones have a hull code and planetary-based ones use "body strength." They are comparable in size to Star Destroyers, but they aren't armored like them. Shields are available, but not massive ones, and backup shields are something only the Mon Calamari have. Space arcologies orbit their star or are in deep space, and do not have drive systems.