How to install certain modules only for specific site using Acquia Desktop
do i need to first manually create a modules folder inside the site folder such as practice.localhost\modules (where practice.localhost is the site name) folder as the modules directory is not created by default for any of the subsite folder.?


Yes, you need to create a specific modules folder for the specific site(s) you wish that module to be available for. If you want it available on, say, 10 out of 20 sites, you'll need a copy in each site's modules folder. You can remove the need for this by making it available in sites/default/modules, but obviously it will be available to all sites in the multisite.


  are going to find it difficult to export sites to FTP for making the site go online.?


Completely depends on your deployment process. You'll probably want to review that process before implementing this on your live servers, but whether or not it's 'difficult' depends on what you need to do.