I don't think your tank has enough decor/plants inside of it for 5-6. I would put in 3-4 DPs and see what would happen. DPs mostly hang out in the bottom-mid level of the tank, so maybe you can fit 4 with your biocube's floor plan (or footprint).
DPs are only picky eaters only at the beginning. They need to adapt to eating dead food because they eat live food in the wild. Plus hobbyist tend to cram them inside a tank. If not enough space in tank to established territory, then they are less likely to eat.
All you have to do is add 1 DP at a time. When you first get your DP, treat him/her with internal meds everyday for 1 week. Then try different food until they learn to recognize you are trying to feed them and they learn what is food and what is not food. Once you get your single puff to eat most things, then you can introduce another DP. You do the same thing with the second puffer as you did with the first puffer.
I think the above process will help you train/treat each DP. If one of them won't eat, then put that puff in a bucket (or anything that can hold water, but another empty tank preferable) with decor and airstone. Isolation will give them a more comfortable time eating and living.
After you get over this hump, then it will be easy sailing. Many sources states that DP need hard-shelled food to keep their teeth trimmed. These sources are false. DPs do not get overgrown teeth. Feed them raw shrimp, dead or live worms, and pest snails you find at your LFS.
This was a brief overview. Post if you need more details or if your run into trouble. Good luck.
