Old Dog Fighting With New Dog

[LexiAndMonty]

I've had Lexi (grey) for 4 years, she's 9 now. She's a rescue and wasn't socialised and didn't like affection when we got her. She's made a lot of progress and she's made a few friends at the dog park too. She still doesn't like all dogs but quite often she is friendly to new dogs. She also gets on great with my sister's Yorkie. We decided to adopt another husky, Monty (black) (4 years), and they seemed to get on fine when they met. Lexi started becoming quite possessive and food aggressive, which we seem to have rectified now at least. She's learned to share her humans and food isn't a trigger anymore either. She mostly just ignores Monty now but sometimes they even cuddle so I don't really know what's going on. Other times she growls when he gets too close or tries to play. We also go on a walk every day where they're completely content walking side-by-side. Monty isn't completely socialised either but is a lot better with other dogs. They only occasionally get into fights now but I can't figure out what the problem is now as it only happens once every 3/4 weeks more or less and I cannot figure out what's causing it as I never manage to catch the beginning. Does anyone have any tips on stopping the fighting and getting them more friendly towards each other?