Mixed stands of uneven age species of trees make the most sense fire wise. This also makes great sense ecologically.
Diversity promotes stability where monoculture promotes instability.
To determine tree spacing, as a general rule of thumb, take the diameter of the tree at breast height. Lets say 36 inches. Take one third of that, or 12. Convert to feet, and that would be your radius for spacing.
So a 36 inch diameter tree at about 5 feet from the ground (DBH or diameter at breast height), would need to be spaced approximately twelve feet.
With uneven age stands, which you want, you would go off of each tree’s specifics to come up with general spacing ideas. As trees get bigger, you thin them out.
So that same 36 diameter tree may have younger trees closer in than twelve feet under its drip line. This mimics the natural forest process of uneven age stands of mixed species of trees maturing and replacing mature trees with young trees over the years.
Firewise®, limbing trees up one-third of their height removes ladder fuels that could potentially carry fire from a ground fire to a crown fire.