I was wondering if I can graft plum vareities to a peach tree, I know they are different but I remenber reading that they are close enough for grafting. Although I do know that peach cannot go onto plum rootstock.

Some peaches can go unto plum rootstocks. The same also that some plums can go unto some peaches.

If you try Krymsk 1 rootstock, it will accept plums, apricots, peaches, nectarines, pluots, apriums, plumcots, cherry plums, almonds without any complaints whatsover.

Sure you can.
Here's the original green gage plum that I grafted to a peach tree and if I did not keep on getting scions from it to graft from other plums it would have been much bigger.
The problem is that this plum is taking over the peach tree. It is sucking all the juices from the peach tree so that the peaches (3 different varieties in one trunk are dieing ( branches drying).
I used to have very nice fruits harvested from my peaches but when the green gage took over the fruits were small and scrouwny. But I love the green gage so much so sweet and juicy I did not care what happen to the peaches. Now I have this green gage grafted so many times to my satsuma plums and santa rosa plums.

Thanks guys and benny now that you tell me this I might graft my plum to some rootstocks.