Plant,
Masters classes have risks and rewards. The risk is that most masters classes do not adhere to the 80/20 principle. It’s closer to 50/50. That’s OK, if you can adjust outside the swim class your other volume to compensate. Certain athletes also have trouble in a group environment in swimming easy, even when the segment is supposed to be easy: it’s always a competition, so it ends up being closer to 25/75!
But, as you have pointed out, you can get great feedback and observe other great swimmers. If you can overcome the intensity balance issues, this is a great option to improve technique.
In terms of how you incorporate it into your plan, you can simply substitute distance for distance. If the plan calls for 6,000 yards a week, you can fill that with masters, and the day of week can be flexible. Swimming recovery is so quick that it really does not matter as long as they are somewhat spread out.
Finally, you don’t have to meet the exact swim distance. If masters class has you swimming a bit lower or higher than the plan, don’t worry about making it up. If you are within 85%, that’s great.
David