No, you can't add a DP. It would probably shred the mollie's fins and/or be eaten by the crabs. Also, be careful with crabs & ADFs. You might wake up and find you have 3 mollies and 2 crabs left.
If I were you, I would take the crabs & 1 mollie back and get a school of 6 small tetras or a betta, or I would take the crabs back and get another mollie & another ADF. I've had a red clawed crab twice. The first was years ago when I was quite small, and I had a crazy overstocked tank. The red clawed crab helped bring the stock down. The second time I had 1, I was keeping it in a small container (not tiny, just not big enough) for the time being. I tried an amano in with him, and all you could see was a blur of shrimp and a flailing crab. I find red claws very agressive, and I wouldn't reccomend anyone keeping anything slower than a danio with them.
Also, whenever I see red claws, their are a few crab appendages lying around, maybe a shredded crab or 2, and 2-3 living crabs. Don't be surprised if you end up with 1 crab that killed and/or ate the other.