Welcome to our forum.
"I'm new here. Is it possible for a mature single Scottish woman to explore polyamory?"
Sorry, exploring poly is only okay for the rest of the U.K.; it is naughty in Scotland.
Yes, I am kidding ...
You're barely older than me and I'm in a poly V (the couple I'm with is married to each other), so you are certainly fine as far as age is concerned.
"How does this all work, though? How does everyone keep a healthy emotional balance?"
Communication, communication, and communication, as we so often say in "the poly world." Not just high-quantity communication, but high-quality communication as well. It would probably take a million years to master good communication, so just assume you can always get better at it with more study and practice.
Some of the most important points of good communication are, putting yourself in the other person's shoes, listening as much as you talk, and not assigning blame, neither to yourself nor to others. You might want to schedule weekly or monthly sit-downs with this couple you're interested in for the first few years of your relationship.
In addition to communication, keep in mind that equal isn't necessarily identical to fair, nor vice versa. All people are unique and have their own individual wants and needs. So what works for one person doesn't necessarily work for another, and sometimes one person needs more time or attention than another. This isn't a plug for self-sacrifice, it's just advising you not to try to make everyone's situation identical.
It sounds like right now you and this couple are just in the "friends zone." So let's start with that, and if things get more involved, we'll figure out how to handle it.
Keep us posted. Glad you could join us.