I am currently half way through my node.js project and now, I want to install npm-shrinkwrap for dependency management. SO is it ok, if I do it now, or I should not do it right now.
You can add a shrink-wrap to your project at any point, so yes, it's OK.
As a side note, you may want to take a look at yarn, which is an alternative dependency management tool which does a much better job at locking down dependencies than npm-shrinkwrap.