Do not leave copper pennies to soak with steel pennies. The copper could react with the cleaning chemicals and somewhat dissolve in the solution. The dissolved copper will bind to the steel pennies, ruining their finishes.
Toothbrush bristles come with varying degrees of firmness — you want one labeled as soft or extra-soft. If you use a firm bristle, you risk damaging the coin.
Consider adding salt to your vinegar/citric acid/soda. The combination of acid and chloride may make your cleaning solution even more powerful.
