Can any stolen military vehicles be insured/garaged

You can still upgrade the Sports cars like the Inferno, Banshee, etc. However, you can not upgrade them if you stole it. By LS Customs telling you the car is "too hot" it means that it's a Super Sports care that was stolen. To upgrade one of the vehicles, you have to actually spend your money on the web and purchase them. No worries though, buy insurance and a tracker for them and you will always be able to get your vehicle back even if it blows up.

Installing a tracker allows you to see your car on the map. You can install a tracker at LS Customs, or you can get a free one when you store the car in your garage.

A tracked car can still be totaled or stolen. If this happens you are out of luck. Buying full coverage on your car at LS Customs gets around this. If the car is lost or stolen you can call up the insurance company and file a claim to get it back.

Each garage can hold a set amount of cars(between 2 and 10). Most cars you can steal can be stored in the garage. The game is a little restrictive on the high end cars-you have to buy them. I haven't tested it, but I assume you probably won't be able to store the bigger vehicles like semis and cargo trucks either. Physically they are probably too big to fit in the internal volume of the garages(at least the 2-car one I currently own)

Another bonus of the garage is you get a mechanic who can deliver a car from your garage to you anywhere on the map for $50.