That's interesting as I am finding plum a bit difficult to graft .
I certainly don't think it will make the tree more likely to get ill in fact I suspect it may be very healthy
Plums are usually grafted on to ... Plums

abet ones that are smaller than the preferred plums will grow into plus they will fruit quicker .
What type of plums are they ?
For hardwood cuttings I usually use quince as it's very easy and I can graft apple and pear on to it .
David
Living in Anjou , France,
For the many not for the few
http://www.permies.com/t/80/31583/projects/Permie-Pennies-France#330873