The 77 gal Hagen (OD 48 x 16 1/8 x 24) I purchased used has a rim but no center brace. Being a newb I had no idea whether it was supposed to have a brace or not. I've recently seen several new 77 Hagens exactly like mine and they were exactly like mine, rim, no brace. I actually added a ghetto wooden brace to mine soon after I purchased it since I noticed a bow in it when measuring for a canopy. Needless to say, it was not a perfect rectangle. I've decided to leave the brace on since I personally feel uncomfortable with the bow in the tank sans brace. The glass thickness is 3/8" and according to Hagen, none of their tanks use any tempered glass. Hagen's been in the business a few years now, so I reckon they know what they're doing.
With respect to your own plans, I have no experience with Aqueon tanks, so if I were you, I'd check with Aqueon to find out a little more info about their tanks. I found the info regarding non tempered glass on the Hagen web site FAQ. I'd say it's pretty much a sure thing that your tank will bow as well once you remove the rim/brace, even if only a little. How much depends on the glass thickness. If you don't need a perfect rectangle for a hood etc, it might not be a problem on such a low tank. Proceed at your own risk though, and I reckon the company will advise against it as well.
EDIT: Link to your product manufacturer's FAQ
http://www.aqueonproducts.com/educat...quarium frame?