Are these M.Hostillis or M.Silk Tree?
I can't tell yet.

What are the defining characteristics of M.Hostillis?

These look like Albizia Julibrissin, the persian silk tree.

If you germinated them yourself, the easiest way to tell is looking at the seed. Albizia seeds are much bigger, with elliptic shape. In the seedling stage, the cotyledons (the first germinal pair of leaves) are thus much larger, and the first set of real leaves is a little larger too. Also, the leaves are terminated in a slightly pointy shape, unlike Mimosa Hostilis leaves.

Seems that a few suppliers are selling Albizia seeds as Mimosa Hostilis... for seed ID, you can take a look at this Seed Gallery.

Thank you very much. I'm going to try to find a more reliable vendor for seeds.