Piccolo is like Krillin, always going to help regardless of how he compares to the threat. He was going to interfere against Perfect Cell before Goku stopped him, and he already knew he was complete trash compared to that level. Him wanting to save the duo doesn't weigh heavily for me in regard to his strength. Nail's praise says a lot more than that.
I usually have Gohan slightly stronger than this Piccolo, with Krillin lagging a bit behind Piccolo. They're all within range of each other to give a good fight, though, so with two against one it's a loss for Big Green.