You’ve got the right idea. Give it a fast draining soil mix with slow release fertilizer. Potted Mangos will perform much better if given occasional applications of minors and trace, especially Iron, Zinc, Magnesium and Manganese.
Don’t over pot it, put it in a container that is slightly larger than its root mass. Once it fills in that pot with roots, up pot again with a slightly larger pot again.
In SoCal, mangos do most their growing in the the heat of the summer so you need to maximize growth between June-September by ensuring they are properly fertilized.
Right now is prime vegetative growing season for Mangos but since you just received your tree, you have to be very careful with over fertilizing. If you don’t disturb the roots much, you can use a mild fertilizer but if the plant was rootbound and you have to trim the roots, don’t fertilize it until it settles in.
This time of year, Mangos are very resilient with very little signs of diseases because the heat pushes growth and the growth out competes the fungal diseases and die back.
Now is also a great time to graft.
Unlike growing Mangos in Florida and other prime mango growing areas, you want your branches as vertical as possible.
One of the big mistakes SoCal mango growers do is to bend their branches horizontally. Horizontal growth will tend to flower and we don’t want that for young non established trees. Vertical growth has a slight less tendency to flower but only slightly. Everything you can do to tilt the growth towards vegetative growth instead of blooms will be that much better for your tree.
Simon