Hi,
Answers to
1/, no it will not affect your system as these cards are hot pluggable, and as long as you insert into the right slot (refer to the Cisco documentation)
On this Cisco web page, http://www.cisco.com/en/US/products/hw/optical/ps2006/prod_installation_guides_list.html
2/ you should not for minor card addition like this.
3/ you will find the procedure document for the applicable rel of ONS software you are running. The procedure will have a section that documents steps to install various card type.
Hope this helps.
cheers,