Hi,
If I understand you correctly, you custom types will not lose any of the parent type properties. Say you have a custom type (invoice) which the parent is "cm:content"; which has amongst other properties "cm:modified". Now you "invoice" type will by default inherit "cm:content" properties and you custom properties will be attached. So in effect no extra coding; however in your form config "share-config.xml" you will have to explicitly list all properties for them to appear when document properties is invoked.
Hope this helps.