For pipes in series, the minor loss coefficients should be added. The differences in diameter between the original pipe and the resulting pipe should be negligible. You should be given the option to ignore minor losses in series pipes.

For pipes in parallel, you should be given the option to ignore minor losses, not skeletonize pipes with significant minor losses (e.g., if total Km > 100) or account for them as a change in diameter.

One possible short heuristic for handling minor losses in parallel pipes is to realize that you are splitting the minor loss over two pipes. If the pipes are roughly the same length, roughness, and diameter, then the minor loss coefficient will be cut approximately in half. I worked through the math for coming up with an equivalent minor loss coefficient and it’s a mess. Using half the minor loss coefficient isn’t exactly correct, but it pretty much accounts for things.