I think the 12" would be absolutely perfect. But a 16" (isn't the tank 16" wide?) could also work for you.
It's pretty easy to cut light levels down with some fiberglass window screen or other light-filtering material used in photography. So if you find you've got too much light and don't want to raise the fixture up, you can put some strips of material beneath the light to get things where you want them.
But the Edge is shallow. Add in substrate and things are even more shallow. The Edge is only eight or so inches deep/tall, if I remember correctly.
Yup, its about 8 and some inches tall. Of course you have to also account for the 1-2" for the light's legs (assuming it has them).
Somewhatshocked, would 30 PAR be good for shorter, more compact growth for low-light plants (e.g. crypt parva and red wendtii).
I'm still unsure about if the Fugeray's 30 PAR or Ray IIs 49 PAR is better for a non CO2 and simple ferts setup.]
To Soup, I think an upgrade is worth it. Well lit Edges look amazing and many people have maintained low maintenance edge's with upgraded lighting. You could always diffuse the light it if it gets to be too bright. I think the stock light makes for a darker, shadowed tank (which is fine if you like it that way).
If you get the 12, you may not have great light spread unless you hang the fixture above the tank. I think that's all you'll need but some others will likely chime in.
STS: I was surprised by the intensity of the FugeRay. For instance, the 20" is so much light on a standard 10gal tank that I have to use a few layers of window screen to dampen things so I don't have algae issues.