Citrus zest into the end of a boil isn't a bad way to start with fruit flavours, I reckon, including how they can interact with malt, yeast & hops. I've had a couple of goes at using rhubarb too - not entirely successful, those. Easy to get too sharp & astringent if you are working with fruits that can contribute tannins or other phenolics. For that reason, I'd be a little careful with plums. I'd try them in something not too wheaty and if the skins were too tannic, I'd blanch, peel and take the stones out (stones of stonefruit will give flavours of their own - think almond/marzipan/bitter almond/cherry brandy type flavours).
I freeze fruit and then thaw before adding, either at the end of the boil, or into the fermenter when pitching, or when racking to a secondary. For example, I've got a Belgian Cherry Dubbel finishing in the fermenter now. That was backyard fruit picked last summer, frozen, then added at the end of the boil. I meant to use a pectinase enzyme to deal with some of the haze risks of putting fruit into hot wort, but forgot. Unless you are working with really high pectin fruit, or really keen on clarity, you can probably avoid adding pectinase.
One thing I'd like to do but have not yet is use blueberries in a red ale recipe for both flavour and red colour. With plums, I'd be tempted to take a golden ale type recipe that fits well with a hop like Amarillo and use the plums in a secondary to complement the hops, maybe with Marris Otter on the grain side for something biscuity.
Best of luck.
