I know in my company for the same job grade at the same location

(we have branches in more than 100 countries worldwide), the salary can be different as well, but will be within certain range. So two employees who have similar background and working experience are not necessary making the same amount of the money, it depends on the job market when that person was hired as well as that person's negotiation skills. Normally you still have some room to negotiate the salary when the company offers you the job, in general, company would offer you a relative lower starting salary to save their cost.

In terms of firing those two employees, company has the right to do so if they have clear policies regarding this matter in their business conducts.