At this point neither Goku nor Piccolo knew about the moon or the tail being vital to the transformation so they'd lose badly. Raditz would be stronger than nappa with Great ape with 12k-15k power level.
You are mistaken. The Saiyans can't just grow their tails back whenever they want to. The tail can randomly grow back at times, but the Saiyan has no control over this. Also, Goku in particular could not grow his tail back as an adult because he had his tail removed by Kami. Kami, as a god, was able to remove Goku's tail in a way so that it wouldn't grow back. This was because they did not want Goku raging out as a great ape. The reason why Goku's tail grew back in GT was because the elder supreme kai (who is a higher god than Kami) had Goku turn some grinder or whatever, and that somehow caused Goku's tail to grow back. The point here is that the elder kai is a higher god than Kami.
Anyway, long story short: The tail thing is not a shaky thing. Saiyans can't just grow it back whenever they want. Even if they could, Goku could not since his tail was removed by Kami.