Rikti Invasion Mechanics
"During the second phase of the invasion, the Rikti check to see if more of their Invaders are required every 6 seconds.
The Rikti find out how many Invaders are currently on the ground. If that number is less than 100 and less than 3 times the number of players in the zone, the drop ships look for likely places to teleport more Invaders.
To find likely targets for the Invaders, the Rikti pick a hero or villain at random from all those in the zone. If that person is not moving faster than a sprint, at more than 20% health, are not under cover, within 100ft of the ground and are not within 300ft of the hospital, this person is a good target. The invaders will pick up to 5 unique player targets in each search period.
When a good target has been located, the Rikti count the number of their Invaders within 30ft of the target and assign a threat rating to each Invader. Minions Rikti are worth 0.33 point, Lieutenants are worth 0.75 points, Bosses are worth 1.5 points and Heavies are worth 3 points. The threat ratings of all the Rikti Invaders within 30ft of the target are added together to get the current Invader Threat in the area for this target.
Then they count the number of players within 30ft of the target and this number is used as the current Human Threat in the area for this target.
If the Rikti Threat is less than the Human Threat, Rikti Invader reinforcements are sent to the area around the target.
If the Human Threat is greater than 6 times the Invader Threat and there are already some Invaders in the area, the Rikti will send in Heavy Assault suits. They will send one suit if the Invader Threat is less than 1 suit, otherwise, they will send 2 suits.
If they did not send Heavies, they will check to see if the Human Threat is greater than 4 times the Invader Threat. In this case, they will send an amount equal to the difference between the Human Threat and the Invader Threat divided by two up to a max of 4 units. These units may be Chief Soldiers, Chief Mesmerists, Rikti Priests, Rikti Monkeys, Drones, Conscripts, Infantry, Guardians, Headman Gunman or Chief Mentalists.
If they still have not sent troops, they will send an amount equal to the difference between the Human Threat and the Invader Threat up to a max of 8 units. These units may be Rikti Monkeys, Drones, Infantry, Conscripts, Guardians, Headman Gunman or Chief Mentalists. "
Regarding Stealth and Invasion forces, also per The Dark Watcher in the referenced thread:
Stealth powers have no effect on the Rikti's ability to select a Hero or Villain as a target and their counting scanners for determing threat levels are likewise not affected by stealth.