i've been fine with just a molded piece on my upper teeth. in my experience, grappling, you're in much more danger of lacerating your lips than losing any teeth, and covering your top incisers does a lot to stop that. for the dirty boxing and clinch work, or even full blown mma (without headgear), i'm also usually fine with just the one top-piece. unless you guys are throwing very aggressive live knees or soccer kicks, i don't think a bottom piece is necessary.
Survival is Triumph Enough