One of my least favorite Skimmias; in fact would call it a bit of Mickey Mouse plant but if you must then somewhere else in the garden and in a pot. It will need to have light as otherwise you will start to lose the golden edge. Even reevesiana which is an hermaphrodite (both male and female flowers) wouldn't be my choice. Try Skimmia japonica 'Redruth' also hermaphrodite if you only grow one but loads of different varieties if you grow more than one.

Sorry but ts highly recommended that you leave a bare area under young trees for several years at least, so try to be patient- you'll want to give your young tree a good start without any competition. If you fill the area with young shrubs they will take the nutrients your plum needs while its establishing a good root system. In any case three mature skimmias are far too many for that area.

You could pop in a selection of bulbs flowering at different times if you want something there.

Agree with Steragram - nothing should be planted around the tree, preferably within 3 feet, but certainly 2 feet, whilst it gets established. If its on dwarfing rootstock such as Pixie, an area of 2 feet should always be left clear around it - easiest to cover that area with organic mulch.

I like Stera's idea to use pots for now. Obviously you'll want sun plants for now until the plum tree matures. If you haven't already done so, I highly recommend that you plant a 2nd plum tree approximately 25 feet away. Many of them require cross-pollination to produce fruit. Make sure you choose a variety that's compatible with the tree you already have (they blossom together). Cross-pollination also seems to be better for the general health and vigor.

Thanks for all your comments. I will definitely try to put this into practice :)