Re: Use NewCrossFitting() to connect four pipes which in different types,the new family instance of a cross fitting belong to which familySymbol?

I believe you can simply place the desired fittings first and then connect their connectors to automatically generate the types between them. Read the series of experiments to implement a rolling pipe offset:

http://thebuildingcoder.typepad.com/blog/2014/01/final-rolling-offset-using-pipecreate.html