Am I correct in assuming a switch inside the house will allow me to add a tuner without adding extra cables from the Superdish?
As I understand it, the SuperDish will come with a DP34 switch. This switch will be a REQUIREMENT for all installs - even a single receiver. There are three separate LNBF's on the SuperDish. You will need three cables from the SuperDish to the DP34 switch location (preferably inside). From there, you will need one cable to each tuner (ie. 2 cables for 522, 721, or 921).
You can then easily stack multiple DP34's together (via dedicated cascade ports) to feed 8 (with two DP34's) or even up to 12 tuners (with three DP34s).
In the future, the DP34 can be replaced with the DP+44, which will enable you to add another orbital location (61.5, 148, or 121), and will also allow you to use a single cable to feed a two-tuner receiver, with an adapter connected to the receiver.
In short, to future-proof your installation, I'd plan to locate the switch(es) in a central location, indoors, and run four cables out to your SuperDish (three for use now, one more for a future second dish).