Pectin is used to thicken jam/jelly, you use pectinase/pectic enzyme to break the fruit down, get more juice, prevent a pectin haze; definitely not the same thing, do not ever add pectin to wine unless you are making jelly (wine jelly rocks). And any time you heat the fruit you risk 'setting the natural pectin', so you want to be sure not to skimp on this enzyme when you steam juice. Your LHBS should have all the basics to get you started.
I would not recommend boiling your fruit. Outside of adding hot water to a dried fruit very rarely would you ever want to cook/boil your must. You can pour boiling water over fresh fruit to scald it and in essence hope to kill off any bugs, the ones you can & cannot see.
Plum wine made from all-fruit no added water should be quite nice. When you steam juice you are pasteurizing at the same time, so you can literally store this hot juice in glass canning jars until ready to use, as long as your jars were clean/hot, etc..ie canning process. But your juice is just waiting to have its SG taken, some sugar to reach SG around 1.090, 12% ACV, plus pectinase and yeast nutrient, wait 12-24 hrs and add potassium metabisulfite aka Campden or K-meta/KMS, wait 12-24 hrs and pitch your yeast. Read your pkg labels, they tell you how much to add per gallon, amounts can vary per manufacturer.
As you get more experience you will find that many freeze/thaw the fruit for 1-2 cycles and then start the winemaking process since freeze/thaw will expand/rupture the fruit cell walls giving you more of a yield on juice, even when steam juicing. Then the addition of pectinase increases that yield..all without water being added.
As mentioned plums can be acidic, and it really is ideal if you can check the pH. A digital pH probe is better than pesky paper strips, and a basic hydroponic/fish tank digital pH monitor will get you started, el cheapo $8-15, just be sure to calibrate the thing. You can also buy acid test kits at most LHBS or online which check another acidity component, called TA, titratable acidity. You can deal with pH/TA in different ways at different times, come back to us with your numbers. But buy some calcium carbonate(aka chalk) and potassium bicarbonate(I prefer k-bicarb) when you get your supplies, as these are among your options. Better buy a 2nd hydrometer if you can, they break at the most inopportune moments. Grab some acid blend too, or one malic acid + one tartaric acid & you can make your own acid blend if/when needed. (And in about 9 months you will need potassium sorbate, if you plan to sweeten the wine up.)
Since the dominant acid in plums is malic you may want to consider a wine yeast known to metabolize malic acid, like this one
http://www.lalvinyeast.com/71B.asp, it has a temp range of 59-86F and an ACV threshold around 14%.
If you have not yet read over some basics in winemaking, terms, process, etc. I highly recommend starting with
www.winemaking.jackkeller.net, he gives a great overview, has sections for beginner to advanced, recipes and a blog. Very informative.
Question: how many pounds or gallons of plums did you steam juice to get 5 gallons of juice?