I've just bought a Japanese black pine as a bonsai starter from a bonsai nursery and is about 15 cm high. It has a fair bit of foliage but of different size needles, it's only just been potted in and hasn't received any training as yet. My question is: Should i remove the larger needles to get a bit more definition or is it still early days?

I'd let it set root before doing anything else.

Pines recover slowly after any traumatic experience like repotting. You really should let the tree rest for the rest of the year and start training next spring.