Question about Small Schooling Fish (tetra/danio)?

I just set up a new tank 4 days ago. I've added one fish per day like the store clerk told me. Now i have one black skirted tetra, one zebra danio, and one clown loach. Tonight they are very active and chasing each other around. The danio seems to be the one doing most of the chasing. Should i get a couple more danio so he has some fish to school with? Will different species of small fish school together, like if i had couple of different tetras and danios? And my last question... are they being mean to the loach (i didn't know he schooled, i thought he live on the bottom.. he seems to be running..)? Sigh. This is hard work.