Clustering with custom criterion (minimum cluster weight)
This does not work, and it's not how hierarchical clustering works.

If you stop at $n_\min$, no cluster will be larger than $2n_\min-2$ but there will either be plenty of badly clustered points, or unclustered points.

Consider the data set 0 2 3 5 with nmin=2. The first merge is (2,3) and fulfills the stopping criterion. So either you cluster this as (0), (2,3), (5) or as (0,5), (2,3) neither of which is convincing: either nmin is not a minimum size, or the clusters can be arbitrarily bad (and still may be below the minimum size).

The same concern applies to a weighted version.