A tandem breaker is something that is made to fit into a standard breaker slot, but it has two breakers in it. If your panel is full up, you can use one to create another circuit and there is nothing inherently wrong with doing this if you follow local rules. (Read more here
.) Hard to argue with an electrician who has seen the panel when I have not, but I don't think you could "pull too much power and potentially damage the entire panel" since the master house breaker would trip before you did that. You might want to have a home inspector look at your setup and tell you if you meet code for your area.