Is it possible to have both appengine fleixble and standard in single project
There is an even easier way to do this that doesn't require creating a separate service :)
Since you are only developing your second application, you do not need to set it as the default version to serve traffic to the public.  You only need to access it yourself, (or perhaps whoever you give the URL to).  The only drawback really is that you will not be able to access your project at the default url, usually .appspot.com
Therefore, you can simply deploy your flexible app engine project, but make sure you don't give it any traffic sharing or promote it to the main version!!  You can then access your version simply by clicking on the version in the google cloud console, or by visiting http://-dot-.appspot.com/ (It was not possible to access individual versions like this for flexible for quite some time, but this has now been resolved.)
I have an standard appengine app currently running. I am currently developing another flask app which will use flexible runtime. I am wondering is it possible for me to have both apps in same project?