"may i just user one outer ring or inner ring to make a DPT ring"
As per my knowledge DPT will not work in one fiber. As data flows in one fiber and control in other.
As far as i know even with POS card you will not be able to form ring with just one fiber.
WHat you can do is if you do not need protection and have only one fiber for connectivity. Take WDM based single fiber solution means Transmit on one wavelength and receive on different so this way you have bidirectional communication without protection. Only thing whether such thing is available or not for the product you are looking at.
"can i use DPT line card as a normal pos interface '
You can use DPT card as POS if it is made like that such as
this card for 7600 is both DPT and POS
http://www.cisco.com/en/US/products/hw/routers/ps368/products_data_sheet09186a0080088774.html
one more which can be used by both DPT and POS
http://www.cisco.com/en/US/prod/collateral/modules/ps6267/product_data_sheet0900aecd803398df.html
But this one cannot be used for both as it is made only for DPT
http://www.cisco.com/en/US/prod/collateral/routers/ps167/product_data_sheet0900aecd8041add8.html
