They would not go well together.
A) as stated, BNs are tropical and need min 18+C
B) Axies would eat them I think - or try.
C) Despite seeing "coldwater guppies" for sale, they are tropical. Again, not below 18C
If you want fish in with your axies, perhaps leopardfish or white clouds would be the answer. We need someone who keeps axies to tell us if they eat all fish or just slow ones