I would find another location. The maple roots will compete with the escallonia and the maple will win :-) And while escallonia will tolerate some light shade, too much shade - like under a tree canopy - will make for very leggy growth and litle flowering. Escallonia is really a full sun plant. And a pretty good sized one at that - 5-6' tall and equally as wide is common but they can get much larger - 8-10'.