How do you know if / where you'll have scalability problems
You won't.
Just code it for the best model for your data. 95% of the time a RDBMS will work just fine for your data. If you have scaling issues down the road, chances are you'll be working on this full-time and will have the energy to devote to properly thinking about scaling. You aren't Google or Facebook (yet). Until you are, just work with what best fits your data or is fastest to code.
You won't.
Just code it for the best model for your data. 95% of the time a RDBMS will work just fine for your data. If you have scaling issues down the road, chances are you'll be working on this full-time and will have the energy to devote to properly thinking about scaling. You aren't Google or Facebook (yet). Until you are, just work with what best fits your data or is fastest to code.