Can I use an old 50 amp circuit that was previously used for an electric stove to feed a new sub panel in my garage?
To directly answer your question: yes, you can use a 50 amp breaker for another purpose. 

Also, just to be clear, the total of all breakers in a panel can be higher than the main breaker protecting them -- it just means you can't have more than 100A being used at the same time. Most people don't generally use their stove, dryer, hot tub, furnace, table saw and have every light and electrical appliance in the house on all at the same time anyway, so usually this isn't an issue. 

For your garage, you also need to be concerned about the size of wire, and physically getting it to the garage. There are lots of good questions on this that I'd recommend you browse through: https://diy.stackexchange.com/search?q=detached+garage+subpanel