Which base for Carnation pink + Cerise?

It depends on the shade you want. If it is a deeper pink, more towards cerise, you can get away with a darker base, like a ginger/gold, but the more orange in your base the more the pink will pull towards red.
Carnation on its own is a medium pink, so it needs a blonde base. if your hair is yellowy it will come out more of a peachy pink.
You don't need a perfectly pale platinum base for pinks, but the base colour does affect the final result.

Thank you! One more question: I have some violet too. What would happen if I mixed violet with carnation?
And, otherwise, I think I could use the violet to tone my hair before using carnation if I'm correct. Anyway I would be happy with a mix of cerise and carnation, leaning more to the deeper pink and my hair usually goes yellow when bleaching over natural hair (I only have some bits of dyed black hair where I haven't cut yet, the rest is either shaved or cut really short).

Yes you can tone the yellow in your base with the violet, or you can add some to your pink mix, both will prevent a peachy/orange fade. You can also use a violet toner over the faded pink to keep it a true pink.
The darker pinks like Cerise are pretty forgiving over less than perfect bases, so I would bleach and tone, and then adjust your pink mix depending on how pale (or not) your hair is.

carnation needed VERY light base for me to even stay on, anything darker it wasnt visible or wouldnt take. cerise was more forgiving but faded kind of red for me.

It might be worth considering that Cerise may swallow the Carnation so maybe add it little by little if you decide to mix them, same with Violet.

The Violet I received recently looked quite dark, has anyone else purchased/used recently?

Cerise will swallow carnation entirely, it doesn't really bring anything to the table indeed. Makes it more... diluted perhaps.