Multi grafted citrus trees are available at my local nurseries and markets - when picking the strains to graft you need to pick strains that have similar growth habits so that one doesn't dominate the rest. If one does appear to grow too fast it needs pruning to keep it at a similar size to the others or it will take over.
They are popular trees (although expensive) because they give the benefit of several trees for the space usually taken by one, definitely a benefit with urban house lots shrinking in size.
Avocados grown from seed are not the most reliable trees, most commercially available trees are grafted onto vigorous rootstock. A smaller growing tree with more desirable fruit grafted onto disease resistant rootstock makes the better plant - a shorter tree is easier to harvest.
If you do grow an avocado, make sure that it is planted in a well drained area as they do not like wet feet.
I'm hoping for a good crop of avocados this season as my trees have just set vast numbers of fruit and the 10 year drought is finally over ... times of plenty again