You can keep the mango tree pruned to make it grow like a bush, esp. varieties like Coconut cream that like to throw branches downwards make a thick bush in 5-6 years. If you go with tree-shape option for the mango tree then some bushy shrub that likes full shade should be your option because there will be dense shade under the mango tree. Katuk grows into a thick bush, 6ft tall but not sure it likes full shade. Garcinia, seashore mangosteen and Achachairu also grow into a thick hedge 10ft tall, 10ft wide but i have seen them grow well in full sun, may be they will not grow well in full shade. Noni is another tree that grows into a wide bushy tree but likes full sun so won't grow under the mango tree. In my garden Alpinia Zerumbet has grown 7ft tall and 6 ft wide in shade, and roots are superficial so won't compete with mango roots, but they grow fast and there is possibility of it shading the young mango tree. Hopefully others can chime in with their experience.