Electric Panel Capacity

Eng, what's a tandem breaker
My guess: a breaker whose input is from the output of another breaker

...in which case I don't see anything wrong with the practice as long as nothing gets overloaded. But the necessary calculations could be overwhelming

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.

Jason do hear Eng, quite a remarkable fella

Tandem breakers are safe and legal to use only when the panel is designed for them and only in the slots that accept tandem breakers.