This is Q&A system for XC -
https://www.cflsystems.com/product-questions-and-answers-for-x-cart-classic.html . It works as part of XC itself.
I doubt you can make anything work like you say within iframe since the iframe is not dynamic and loads at time of page load. You can make it load different height on page load but you need to be able to read first what the height will be which means you have to access these additional php scripts or info before they make it to the iframe...