An important guideline when coming into a leadership position with senior staff is to take 2 years before you make major changes. The first year you mostly go with the flow and try to fix blatant deficiencies. By the second year you have built trust and a relationship and know the company intimately. Changes can be consistent and more bold by year 2. After year two you can start weeding out those unwilling to adapt to the culture and trend you are creating. If she is going in and making waves I would encourage her to back off and focus on building relationships. Those employees probably know the company better than her and she could use their experience.

It’s hard to answer this question without knowing the nature of your business, but if his attitude is toxic then it will be affecting the work environment and ultimately the customers. At some point you need to assess whether his value to the company outweighs the damage he is doing to the company. If he is a valuable employee with a skill set that you can’t easily replace and he does a good job at a job that is isolated from others, then a bad attitude is something you need to address but maybe not worry too much about. If it’s going to make or break your business you need to do what’s best for your business.