Canary feeding habits

Hello all,

I was offered a couple of two-and-a-half year old canaries as a gift. Apparently, they have never had any cuttlebone in their diet; now that I have some always available in their cage they don't even care to taste it.

Is it OK if I just scratch some cuttlebone flakes off and have them fall in their vitamin dish, so that the canaries may eat it in powder form as they're having their vitamins?

I'm doing so the past couple of days and seems to work. Is there any limitation as to how much cuttlebone they may consume?

Thanks in advance.