Don't think I have time - I'm 76
I was forgetting your age doganjo and, having been reminded, I would remark that any additional apple/s would best be dwarf trees also: these will need to be sourced on-line or at the nursery (as no-knowing what rootstock used for s/market ones).
Dwarfing might not be too great a consideration if you go for a crab merely for ornamental purposes, pollinating-power and bird feed. However, dwarfs are so easy to maintain (prune/spray) and to harvest and also tend to bear fruit after just a few years and will quite easily produce more fruit than you will know what to do with after a few more years.
I'm not well versed on plums accept that they should be summer pruned (not winter pruned).
I believe I am also safe in saying there is no such thing as a very dwarfing plum rootstock.
From my limited experience of actually managing plum trees, I would say that branches are more likely to break (compared to apples) if bearing a heavy crop and Plum Moth can be a real problem: there is a very comprehensive/excellent TAS post by The Balkan Ecology Project under the simple thread title "Plum Moths".
Based on that piece, I would throw in the following thoughts:
if Plum Moth (or apple Codling Moth for that matter) become a problem, a thick loose mulch below trees will aid raking in winter to help birds find any pupae;
I would personally also go for a good spray of organic "white oil" (a simple mix of liquid potassium soap and veg oil) just after blossom petals have dropped as it won't do any harm and might just suffocate a few eggs or baby maggots before they actually get to munch into fruit-lets.
(For old fruit trees, I would suggest a white oil spray late autumn or winter as well with aim of getting into any nooks and crannies in older bark.)