I have a master page that has a lot of topic references. Is there a way to practice all rems that are in these references in a single study session?
Your portals should work; make sure you use Ctrl + Shift + down as previous commenter mentioned.
My preferred solution is using tags. I created a tag called “To Study”, tag any Rem anywhere in my system and then “Practice Document” from that page. Tags are quicker and more flexible than building your own pages with tons of portals. And bonus - when you are scrolling through the tagged items, you can easily remove things from your study page by just clicking the “X” to remove the tag from the Rem. Easy!
For specialized sessions, I keep a Rem called “Study Sessions” and you can add portals and search portals to each session (or even… tag things with the session itself instead of bothering with portals). And of course the main Rem “Study Sessions” is tagged with ##To Study as well so it all comes through if I want to study all. Here is a screencap of my setup:
You could bring them in as portals by clicking the reference-insert into document. Then you could practice the rem inside the document from the top … menu.
I tried this already. Right clicked the reference > Insert Intro Document and it didn’t show me the cards that were due in the reference. Didn’t work! 
Did you ctrl+shift+down arrow in the newly inserted portal?
Thank you this works well! Also by Portal I believe you are referring to a Search Portal, but please correct me if I’m wrong 
Either regular Portals or Search Portals will work as long their contents are expanded.