1 - I keep babies on only a few inches of substrate. Mostly so I don't have a hard time finding them

It is very important that adults have a deep substrate of at least 10". Burrowability (is that a word?

) is one of the key aspects to be considered when choosing a substrate for your monitor. Also, the age of the animal and sex can have an effect on what type of substrate you should use.
2 - You can get one ackie now and add another later, but whether you will have problems introducing them can not be answered until you have them and are putting htem together. It may go ok, it may not. It is common place for ackie that haven't been rasied together to be more incosistant breeders than ones that have been raised with eachother from hatchlings. That being said, if breeding is your goal, start with a pair of hatchlings.
3 - I would strongly advise against an all rodent diet for <i>any</i> monitors of the <i>Odatria</i> subgenus. I feel that 80-100% of thier diet should be comprised of insects (I feed only crickets and pinkies to my monitors).
Best wishes,
JonK