Both forms are valid so you can choose whatever you like the most.
rstudio-server is an independent service not required by shiny-server, it should run in its own port, you should not use the same port for two different services.
Your app folder should be located within the /srv/shiny-server/folder but not necessarily within the sample apps folder.
I think your problem is that the required packages for your app are not installed in the system-level package library, all packages used within your app must be executable by the shiny user. A simple way to overcome this issue is installing your packages as sudo from a system terminal. For example
sudo su - -c "R -e \"install.packages('rpostgis', repos='http://cran.rstudio.com/')\""