Is it harmful to my computer if I subscribe all the plugins to my library?
Nah, just maybe your free time
I’m subscribed to all free modules in the library (with a 10-years old computer) and I think it’s not causing any technical issues except a relatively slow loading of the module browser (by the way that can be worked around through alternative module browsers, see this thread). A very empirical comparison with a default installation didn’t show me noticeable differences in performances. My plugins folder currently takes 1GB.