The shuckle thing was only in GSC. You give a shuckle a berry (i think thats it) and walk 10000 steps and it turns into a rare candy.
Not worth it and not in RSE.
If you have got every rare candies in the game you can get more through pickup.
Have 5 pokemon with pickup in your team (6 if you have a powerful pokemon with pickup).
Repetatively battle trainers and each of the pokes with pickup have a 10% chance of getting an item after you win a battle.
It is better in RS because the levels of the pickupper have to be higher in emerald for better chances of getting a rare candy. ;123;