new 10 gallon planted, ordering plants online, HOW MUCH?

hc will survive. my experience is having trouble getting it to root. i found it very frustrationg. i'm not sure if anyone else had problems with that.

i have no experience with hairgrass

I think that one 2x2 patch of each will be enough. The best way to plant both HC and hairgrass is to seperate it into tiny pieces and plant them individually. They'll fill in over the course of months. Your lighting is really on the low end of acceptible for those plants. With 40 watts of lighting over my 10 gallon, my HC wasnt growing so hot. That was also with DIY CO2. But it can be done. I think keeping your CO2 stable is going to be key.

It's probably too late and I dunno if you have the patience for it (my patience ended after 3 weeks), but the easiest way to set up a new tank with HC and hairgrass is to grow it emersed (no water) on your wet substrate and mist it daily. Both of those plants grow a lot faster emersed than submersed, and you wont have to deal with algae, CO2, water changes, or plants floating up to the surface.

I've never ordered from those online plant stores. I've gotten all of my plants from LFS's or from other hobbyists online. I've gotten the best deals from other planted tank enthusiasts.

PS - I think you should think about going with Xmas moss, Peacock moss, Flame moss, Weeping moss... anything other than java. Java is the easiest to grow but probably the messiest of the commonly used mosses.