If an employer has different plans (the same employer, I must specify); say for example, a 401(k) plan for unassociated-with-the-union employees, a different plan for union employees, and a defined benefit plan (which serves to augment the 401(k) plan), how does that affect the determination of proper coverage?

Any employees subject to collective bargaining can generally be excluded for "coverage" testing and "non-discrimination" testing. The other participants would need to be considered when performing coverage and non-discrimination testing on the various plans.

To elaborate on Effen's answer, the "collective bargaining" exclusion applies to where pension benefits are the subject of good-faith bargaining. Not exactly the same as "union". See IRC 410(b) and regs.