Adding a second filter to the one in the Rio 180 is a good idea. While the Rio 180 filter is excellent in some ways (HUGE biological filtration capacity, plus open access to the air) it is a very low pressure system so doesn't "suck up" much in the way of solid waste. For that, the second filter can be used, at the very least to push solid waste into one corner where it can be siphoned out easily. I'm using the Fluval 104 and it works fine, with more than enough water movement. In fact, witht the spray bar attached, things get very bubbly in there!
If I was keeping figure-8s, I'd probably think along small gobies that could hide among the rocks and Java ferns and stay out of trouble. Wrestling halfbeaks and glassfish *might* be an option, but it depends a lot on your pufferfish. Some people have success mixing figure-8s with other fish, in which case both those species will have the speed and smarts, but it also has to be said others have no luck at all mixing figure-8s, in which case you'd need to rehome the halfbeaks and glassfish.
Orange chromides might also be worth trying, providing there were lots of hiding places. Cichlids usually handle small puffers quite well, mostly by attacking first and asking questions later! Certainly, I've heard of people mixing figure-8s with orange chromides. Again, no guarantees, but worth a shot.
Otherwise, you might try going with lots of figure-8s instead. If you had half a dozen specimens, none could set up a stable territory and defend it adequately, so while they might bicker, none could become dominant. So basically you'd be limited by water quality. I'd suggest at least half a dozen would be fine in a Rio 180.
Cheers, Neale
