I do not believe one can have a 200 Amp sub-panel tapped off of a 200 amp main panel.
You could have 100 sub panels from that 200 amp main.
All that matters is the calculated load.
I was always under the impression that the primary residence MUST contain the main panel and the sub structure contain the sub panel.
No you can put the service panel anywhere on the property.
My answers are based on the NEC, the power company you local area may have other rules.
Don't forget that the wiring to the new residence from the existing garage service will be an outside feeder and must comply with Part II of Article 225 and 250.32.