Beef, how many are typically sent for an invasion? If we take the example of modern day earth with it tech level what type of invasion force should be expected? is it mainly infantry or are they more reliant on hardware like tanks and starships during invasion only sending in troops after the invasion is done to quell the population?
An invasion is usually conducted by a 1-3 frigates or destroyers, a number of gunboats probably around 3-15 and hundreds of fighter craft. This is to protect the major assets however and the main bulk of the invasion force which consists primarily of 1-3 Kill Cruisers and hundreds of dropships.
There are other vessels involved as well, logistics vessels and various other craft depending on the defenses and capabilities of the invaded world.
The Kill Cruisers will open a channel to the invaded world where the Spraug will congratulate the world on being selected for conquest. They'll show a montage of worlds conquered, and those who fell in line flourishing, and those who resisted being obliterated, their populations, and even their worlds themselves being utterly annihilated.
Most worlds in such a predicament will have known in advanced the reputation of the Spraug and the Spraug sculpt this reputation into a psychological weapon. Attacking outer colonies and making a clear statement of intent before ever arriving in force. They'll attempt to get the governments of worlds to capitulate first, rewarding them for doing so with uplifting the civilization and fixing the problems of the world. Disobedience to their dominion is not tolerated however with all forms of resistance being seen as an act of war, to be handled brutally and without mercy, and all conquered worlds are ultimately client worlds to the Spraug Primarcy.
A typical invasion force is not terribly large generally speaking. A Kill Cruiser, though a large and dangerous ship, typically has only about 10,000 spraug aboard but enough space for another 10-15 thousand spraug marines. Spraug themselves have a much more variable size than humans with most Spraug looking quite humanoid, but many being up to large or small size. So amounts of marines can vary.
Each dropship can hold between 20 large to 80 small Spraug, typically. And for an earth sized world of equivalent population and perhaps PL 6 Technology, only a few hundred dropships will utilized.
The marines infiltrating government HQs and killing the leadership and going after military leaders next, and causing absolute chaos within the ranks of the planets defensive forces.
The main bulk of of defense forces will be left largely untouched with only the weapons capable of firing back being crippled. If a large armada is gathered to fight, the Kill Cruisers will unleash their irresistibly powerful Telluric Concussion Torpedos upon them, weapons strong enough to take out even opposing Dreadnoughts in a single shot and Induction Inversion pulses to entirely drain the power supplies of all lesser ships leaving enemy ships stranded with only life support intact and all other systems entirely offline until repairs can be made.
Once the leadership is destroyed, and their main defense force is quelled, they'll crush any who try to take control and any insurgency.
They will use powerful AI to quickly learn all pertinent infrastructural information keeping the society as untouched as possible and simply replacing the leadership with Spraug.
Even money and fashion is copied by the Spraug to keep populations under control and pacified.
Over the next few hundred years the populations are modified via genetic engineering to become compatible mates and the population is slowly bred out being replaced with Spraug, their entire history becoming a footnote in the Spraug gene pool.
Perhaps in a sort of grim irony, or a cruel final mockery, the appropriated history of a population and its various customs and local values become the basis of the various Spraug clans, the Spraug take great pride in "their" history and the clans honor hold a vaunted exalted place in Spraug culture.