Hi, Ken,
As I'm sure you know, in the classic Pātañjalayogaśāstra of Patañjali, sūtra 2.30 lists five yamas, beginning with ahiṃsā. I don't know the Haṭhayogapradīpikā well, but Svātmārāma (or whoever) gives 10 yamas in the addendum to 1.16 (or 1.17 in my 1972 Adyar Library ed.). So even if ahiṃsā is a yama there, as in PYŚ, there's already a willingness to depart from the classic list of yamas, doubling its size. Another item has crossed list in 1.17; thus śauca is a niyama in the PYŚ but has become a yama in the HYP. I'd guess that the yama/niyama (commitment/obligation) distinction wasn't that well-defined by Svātmārāma's time.
As an aside, Buddhaghosa works with five Niyamas (or niyāmas).
In the Milindapañha, niyamas are mostly paired with saṃyamas (not yamas). The lists overlap with Patañjali. I haven't looked at this closely.
Best,
Dominik