How I use my charmander candy?

Impossible to answer right now. We need to know more about the new gym system.

But if I HAD to answer now, I'd use my candies towards the Level 21, because he has a better moveset, and the other one is already a really high level.

If TMs do what we think they do, the 100 IV one for sure. No way you can go wrong with a 100 IV one in the long term.

Do not make any power up decisions until the rework is in, as we don't yet know how motivation and CP will interact. Some people have theorized that higher CP mons will lose motivation faster as a function of their CP or level. Personally I think it will likely be based on species, not the CP/level of that specific mon, but it's important to find out how this will work before making decisions.

Make your own choice. Stop relying on others.

I'd calculate and put aside the number of candies required to max out your lv35.5 to either your current level or Pokémon level 39, and invest the rest in that dirty, dirty lv21

With TM's on the horizon, moves, as long as they are not legacy, can be changed, and that fire blast could change to overheat