If you want to link to another Page, you can either use LayoutId or friendly url names. Both are unique for each companyId, so you are going to be pretty safe using them.
You can set the friendlyUrl as a PortletConfig parameters, so you can set them on portlet Level, and you won't have them hard-coded in your Portlet. Alternatively, you can also save them as custom params in portal-ext.properties (will apply for all portlets of that portal).
Now, that's a lot of code for this, so if you're dealing with specific problems, like creating Portlet Configuration or Reading portal-ext.properties, or creating renderUrls programatically, you should start new questions