Is it safe to Symlink your applications folder?
It would be possible to use the NAS as your applications folder, however you would not be able to create a symlink, as symlinks do not work across different partitions, or network shares.  You would instead mount the share as your applications folder.

There are other considerations you may want to be aware of, the first of which is that most NAS shares using windows file sharing would (samba, cifs) do not use a unix style permissions system, so there may be some issues with application permissions.  This could be handled by using a NAS that allows you to export shares using NFS which allows for more compatible permissions on the files.

The second issue is that of network connectivity.  If there is a problem with the network, your applications folder will disappear, and this could cause instability with your systems.  To overcome this issue, you may want to only mount the users applications (/Users/username/Applications) over the NAS, and not the system applications (/Applications).  This way important applications such as Finder, etc will be local, and other applications can be installed to the users applications folder.  Be aware, that most shortcuts point to the system applications, not the users applications folder (e.g. in Finders Sidebar the applications folder shortcut usually points to the system applications folder, and applications purchased via the Mac App Store install to the system folder too).  Users may still have problems with certain applications when there are network issues, however it will not effect the whole system.

The third issue is your question seems to imply that this applications share will be mounted by multiple users.  This may cause some issues, but applications are not supposed to store user data in the application itself, so you should be fine, however not all applications follow all the guidelines 100%.

To mount a share as your applications folder use the command: 'mount shareIpAddress:/pathToShare /Users/username/Applications'  for further information on automating this, you might want to read the Apple document: http://images.apple.com/business/docs/Autofs.pdf