It's fine for a sump application. All of my sumps are glass tanks with acrylic baffles. Just be sure to get the 1/4" acrylic. I tried it once with 1/8" to save money and had to rip it apart because it collapsed under the weight. I didn't think there would be that much pressure on them. Live and learn.
I am in the process of doing the exact same thing with a 10G. Making a Fuge out of it. I plan to use 1/8 Glass because Acrylic does not bond well to Glass. I have friends that DIY these all the time and this is based on their advice. Glass baffles for Glass Sump, Acrylic baffles for Acrylic Sump. Aquarium Silicone for Glass & Weldon for Acrylic.
Generally speaking,silicon doesn't bond well to plastics long term.If you scuff up the edges of the acrylic with sandpaper it'll work better.Since baffles usually don't have a big load to support its not a big deal anyways.I use 1/8" glass baffles in my sump because thats what Home Depot had.If i was to go to a glass shop,I'd probably just go ahead and use 1/4".