I have a small area in my backyard where I have put in native grasses
(indian grass, switchgrass, little bluestem & big bluestem) It is
coming up slowly, I planted from seed about 8 weeks ago. Now the
tallest seedlings are about 3/4 to 1 inch tall.

Problem is there are a number of weeds in the area. Do I pull them,
or just leave them and assume the grasses will eventually grow over
them and make them go away?

Hand weeding's probably the easiest, especially in 200 sq ft.
Here's some help with your project:
http://dnr.state.il.us/conservation/...irie/table.htm
http://www.inhf.org/prairiemgmt.htm