You can also use Ctrl-M to scale the brush by less-than-1 factors to make it thinner, if you're using JACK. Or you can use its vertex scaling feature in the vertex manipulation tool.
On top of that, the 1 unit gap is much harder to notice than you think when you're not looking for it, and when people do notice it anyway, it's no big deal.
Now, about the decal thing, I only remember that messing with decals.wad is one way to cause similar problems. It is advised never to modify it or provide your own. But that's all I know.
You can achieve decal effects with sprites too, iirc. The303's website has a lot of info about it.