Can I use just canna b when my plants come into flower?

HI, don't use only one component of the fertilizer. ph should be between 5,5 and 6,2/6,4 don't give them 1-week water only, better give them a day lower ec and let the coco dry for 1-2 days. this will add oxygen to the roots and they will get stronger gl&hf

Would a sprinkle of worm castings do as the nitrogen,slow releasing, and the just the PK from the nutrient plus some boost?

Flowering plants need only the slightest bit of nitrogen compared to P and K, with P being the most essential to pack on big buds. I don't have too much experience with Canna A and B, but it is typically recommended to follow the manufacturers feeding schedule at half dose to be safe, and if your plant shows sign of deficiency, bump it up a bit.