OK, getting closer. It sounds like even though I have a cuttler hammer panel which has a max of 10 tandem breaker slots (slotted bar), I can still add a tandem breaker (one that is a non-ctl) to this panel as long as I do not exceed the 42 max pole requirement. If this is the case then I am set to go. I am just confused since I believed you cannot add a non-ctl breaker to a ctl panel. I just want to make sure I do this correctly and not burn the house down. So in your professional opinion, do you think it is acceptable to add a non-ctl breaker to a ctl panel? Also I planned on using THHN number 2 wire for the hots, but what size should I use for the ground and neutral? Thank you again for all the assistance.

As long as no more than 42 circuits, here is no problem, this is not a fire hazard, the breaker is the same only mounting is a bit different.

For a 100 amp feeder, #2 is also needed for the neutral, and a #8 for the equipment ground.

Be sure to separate the neutral and ground at the new subpanel, with the neutral bar isolated/insulated from the metal box, and bolt the equipment ground bar directly to the metal box using machine screws and threaded holes.