Should I evolve these Pokémon?
My nidoran is male and has more than 500 CP, but won't make much headway in battles, would cost 3000 Stardusts if powering it up. The other one is nidorino, just above 500 CP, would cost 1600 Stardusts if powering. Should I wait for another nidoran? I am level 22, almostly level 23. Or I should evolve either of them? Note: I have about 120 nidoran (for male) candies.
Answers
Evolve neither unless you just want the Pokedex entry. Nidoking/queen aren't that useful anyway.
For attacking, their poison typing isn't that useful, fairys aren't that common, and some of Clefables moves are psychic type which is super effective vs poison. Even against Fairys you would still probably do better with a pokemon that does normal damage.
If you get a poison type moveset it will also do more damage to grass types, but the grass types that people leave in gym either have poison+grass typing (victreebel, venusaur, vileplume) or psychic+grass (exeggutor). poison+grass does neutral damage as poison is resistant to itself, and against exeggutor, exeggutor has psychic type quick moves which deal super effective damage to poison, and exeggutor's charge moves are grass or psychic, both which deal more damage to the poison+ground typing of nidoking, which basically counteracts the extra damage that you deal to them, so there's no niche use for a Nidoking.
Also anything that has rock or ground typing is useless for defending, because everyone has a Vaporeon, and/or grass pokemon to counter Vaporeon, both which counter rock and ground.
Nidoqueen/King is worth it with the right moveset IMO. That would be Poison Jab/EQ or Poison Jab/Sludge Wave.
They destroy fairies, as mentioned by someone else in this thread. Clefable having access to Zen Headbutt is not very relevant, as it doesn't get STAB and therefore won't hit Queen/King harder than a neutral move with STAB anyway.
Also, while Queen/King do not hit grass/poison types super effectively, they hit them neutrally, while often double resisting the return damage - both poison and ground has a resistance to poison, giving Queen/King a double resistance. Venusaur's A-moveset for defending is Sludge Bomb to hit fire-types harder, making Queen/King a decent pick.
In addition, Earthquake-variants of Queen/King destroy defending Muk, while taking little to no damage in return due to double resistance to poison. Some Muks may carry Dark Pulse to counter psychic attackers, but the Nido's won't really be bothered by that.
Further, Queen/King attacks well into defending Arcanine, although waters will be even better for this.
To summarize, the Nido-royalty is usable versus a lot of defenders. Both Venusaur and Muk have become more common as more players get access to them. Wigglytuff and Clefable are the best fairy-type defenders and not uncommon either. And there has always been a lot of Arcanines.
