While I've not made plum cider, I have made plum melomel, and I've experimented with prune juice. If you're thinking of fermenting pure plum juice, I can tell you it will be WAAAY too tart. There's a LOT more acid in plums than most people realize, you just don't notice it because of all the sugar in the fruit that balances it.
If you're thinking of adding plums to apple juice/cider, it will naturally add acids too, making it more tart. You can add tannin and/or nonfermentables (maltodextrine) to balance, but it's really about making what you like.
I'd start with 1 or 1.5 pounds of fruit/gallon, that should still let you taste the apple & avoid excess acids. I've used black plums & European (Italian) plums, and have had some tasty results with them. I leave the skins on & get a nice ruby red color from them. I'd wash & stone them, then quarter them. I put them in big ziploc freezer bags & freeze them solid for a few days, then let them thaw in the fridge, then refreeze & thaw them again. Let the fruit come up to room temp before adding it to the must & be sure to use pectic enzyme.
I've also pureed the fruit in the blender for primary, makes for great juice/flavour extraction, but you'll get about 4" of lees that way. IMHO, plums are worth the wait & can be a complex & evolving flavour. Try a bottle at 6 months & then try one at 1 year, you'l see a big difference, though it may seem sublte at first.
Hope you find this info useful. Regards, GF.