So after starting an instance but before opening a d/b, I can select any of the d/b to be opened. Once I open a d/b then I have to shutdown the entire instance before I can open another d/b in the same instance.

Do I understand this correctly !!

I start the instance with the foll:
svrmgrl>startup nomount;

And I mount/open a d/b with the foll:
svrmgrl>alter database mount; or
svrmgrl>alter database open;

How can I select which d/b I want mounted in the instance both on NT and Solaris?