I know I've had this discussion before with other sav owners.
These lizards mainly eat invertebrates in the wild (grasshoppers, locusts, worms, beetles & their larvae, snails, etc). This constitutes about 75% of their diet (not really sure on the statistics, but easily over half). Vertebrates make up most the rest, and include other lizards, small tortoises, snakes, ground birds, and probably the most rarest of these, rodents). In addition to the above, some savs have been known to occasionaly take vegetation, and many breeders have started including this in their animals' diets.
The point is that while the occasional rodent is fine, too much can lead to obesity (like Ztrip mentioned) and also, impaction, because savs digestive tract aren't meant to process large amounts of hair. This is why rats are the worst choice because their fur is thicker and coarser. I would recommend small mice or rat fuzzies for your rodent supplementation. A diet primarily of rodents is like us eating McDonalds and Outback every day.
Also, I've notice with many savs that feeding live mice can lead to a potentially more bloodthirsty and aggressive monitor. At least try to convert him to F/T rodents. And of course, there's always the risk of a rodent inflicting a nasty bite to your lizard.
Jeff, it does sound like your monitor is appropriately-sized for his age. His body should be well-rounded, but not like it's swollen and about to burst. When I had my sav, my diet was:
45% insects (crickets, superworms, canned grasshoppers)
45% canned monitor.tegu food
10% other (either eggs or rodents) In fact, I think in the year or so I had him, I fed him one mouse, only because none of my snakes would eat it. So personally, I think a sav could live a long healthy life w/o eating a single rodent. That's just me.
Here's a pic of "Titan" while he was my ward.
He was in 75 gallon tank but probably would need something larger when he got fullgrown. I used aspen shavings without any major issues.