Answer: These trees will take pruning fairly well, however you might want to look at a book or two about pruning or perhaps consult with a professionally trained and certified arborist who specializes in ornamental trees so you can learn how to do it correctly to preserve the health and natural shape of the tree.
You might also consider that the holly is an evergreen and will be clearly visible as a backdrop to the maple during the winter. In summer, spring and fall, the maple will be a lovely specimen tree worthy of attention whereas the holly will fade into the background naturally. These maples can become spectacular specimen trees. So, in the end, you might not mind allowing the maple to reach its full potential. This would allow you to enjoy some of the dynamics of the landscape and the full beauty of the tree.
If you truly need only a small plant in that location, perhaps you should plant the maple somewhere else. In considering where to plant it, keep in mind that these trees need to be protected from hot afternoon sun, should be sheltered from the wind, and do best in an evenly moist, organic, acidic soil.
Enjoy your new tree!
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