One thing I can say about symbols for sure is:
DON'T ASSAULT YOUR READERS WITH THEM
I've analyzed quite a lot works and it's painful to see when writer use overly reoccurring symbols or those that are forced or do not fit within the work. It is much like humour, you shouldn't have to explain it, too much and too forced makes it has the opposite effect and looks atrocious. Just like road signs, you shouldn't have to explain them (or not explain too much). Not to say obvious symbolism is bad but I'd say it's hard to get it just right, but it does benefit to have numerous interpretations of the symbol. Hence in many situations, the symbols you don't notice may have greater impact on your overall experience and overanalyzing may actually decrease your enjoyment of it, in my opinion anyway.
They also make great use as a tool for foreshadowing as you said (though I prefer to see it the dots that links all other dots instead of numerous dots) but also for things such as personality or more visually in the fandom as cutie marks. Another example of symbolism that I can remember off the top of my head is the characters from Lord of the Flies, in particular the glasses represented clarity among the chaos (don't exactly quote me on that, haven't read it in years)
There is room for obvious symbolism but it could quickly turn your work into something more allegorical like Animal Farm and Lord of the Flies. If that is your intention, go ahead. However it may come off as preachy or overly political as some allegorical works do tend to turn out, and it might turn away readers. It is up to personal preference but it is good to keep in mind who your audience is and what they want. There are exceptions, but I personally think it would be hard.
Just because something is a symbol doesn't mean it is all there is to it. There is always more to what it than what it represents. Take for example, the Grinch steals presents, trees etc, all the symbols associated with Christmas, but there is more to Christmas than those symbols. Just like there is more to love than just holding hands and kisses. Do not rely purely on symbols to convey your message or provide characterisation. Have your pony define their cutie mark and not the other way around. Not to say your story can't revolve around a symbol but don't let it take control.
Another thing, a symbol is not the same as a simile or a metaphor, it often holds much more importance, not that say that a simile or metaphor does not, and it doesn't have to run parallel with a theme but symbols are not to be too carelessly placed. You could get away with using simile and metaphor more often than a symbol. Too many symbols usually isn't a good thing. Too many of anything isn't a good thing. It loses its subtlety and balance.