Let's say for example that I am already using another wordpress polling plugin. Can I use a shortcode from that plugin to display the polling within a topic post so that those who are invited to cast a vote can engage in discussion about the poll before or after their vote?
Maybe, it depends on that plugin. Try to enable "Enable WordPress Shortcodes in Post Content" option in Dashboard > Forums > Settings > Features admin page. Then put that short-code in post content and check it. I'd not be able support if there will be any issue, you should contact to that plugin support team.
I'd also recommend wpForo Polls addon, it's designed for wpForo and works in forum topics.