I have always understund the regs to say each business that you choose to treat separately would need to have a minimum of 50 employees. The business with fewer than 50 employees would need to be agregated with the larger busineess as it does not stand alone as a SLOB,
Since there is no requirement that a qualified plan must accept rollovers into the plan, the plan has discretion as to whether it will accept rollovers and from which class of persons. A plan is not required to accept a rollover from a former employee who has not taken a distribution of the benefits from the plan or from that employee's beneficiary.