The average mature tree can have one eighth of it's foliage removed with no problems. If you must remove more, never take off more than one fourth of the branches and foliage or your tree may suffer from so many wounds that it invites disease and insect attacks in it's weakened state. You can always thin out deadwood or remove a single wayward branch without causing major harm but in general try to avoid pruning large trees. A certified arborist in your area will be the best source of local advice about your mimosa tree pruning. Keep growing, Marianne Binetti