I hope that is not going to be a permanent home for the oscars. I would do two AC110's on a 55. tons of filtration for a 55. I think a fx5 would be overboard.
filstar xp3 is a reliable midgrade canister that work well on a 55 in my exsperience. And maybe add a ac110 latter if needed as a hob filter. I actually used this combo a couple years ago.
I have been very happy with my Rena XP3 on my 65 g. African tank and I love the optional inline heaters that are also the intake tube and can run dry without burning up. Although I have not personally tested the run dry claim. Even though I imagine you get some bypass with the media baskets they work great for customizing the filter media.