I ran a fuge IN the back of an IM 50 AIO last year. It worked pretty well. I had considered a HOB version but ended up with the
refugium caddy from InTank Aquatics. I used a razor blade and peeled away the black backing on the second chamber from the left in the back and put the caddy in there with a light behind the tank. I had some issues getting chaeto to grow early on but I don't think it was related to how I had it set up. I increase the photo period and started dosing ChaetoGrow and it perked right up. I shut that tank down after a year (I was moving) and upgraded to a bigger tank with a sump. I still have a refugium but I used Pom Pom gracilaria instead of chaeto now. I would 100% do that instead of chaeto if I had to do an AIO w/ fuge again.
Unrelated but I'd also highly recommend the InTank
floss holder and filtration cover. Excellent products that really work well with the AIO setups.
Link to my AIO build thread:
https://www.reef2reef.com/threads/first-build-im-50g-aio-mixing-station-qt-setup.961920/