I run three AC 500's (old school 110's) on two different 6ft 125's. I've never had one leak on the floor, and I only clean the media every 3 months on average (all 3 filters at the same time), but my tanks are typically stocked more on the lighter side of the equation, and the bio media alone contains ample bio bacteria to run everything. It's simple enough to crack a lid once a week or so and check on the overall water level to decide when maintenance time is coming up. Don't be like DOOB and find out the hard way. lol
As far as media, I run all my AC filters the same way. Coarse AC sponge at bottom to catch the big stuff, then a layer or two of a finer blue/white media (some media brands are better than others) to catch finer particles, and then a large bag of ceramic bio rings on top. Most of my sponges and bio media rings are approx. 15 years old, or older, and only get a light rinse in dechlorinated water during maintenance. The finer media gets replaced when it starts shredding apart.