Queries regarding Client/Server multi-platform MMORPG app

I'm new to Unity, went through a couple of basic tutorials.
I've been wanting to make a MMORPG game targeting multiple platforms(only iOS and android, for now).
I've done my homework of googling for answers since a couple of hours now but I can't get to the point answers since I have only basic knowledge about Android(Java only) and no knowledge about iOS. Here are my queries-
1. Is it a good practice to use C# for client app and Java for server side?
2. If C# and Java are suitable as my previous question, will my C# code work indifferently for both platforms?
If not,
2.1. do I need to use platform defines like UNITY_IOS and UNITY_ANDROID respectively to compile the codes differently?
2.2. do I need to install any Unity plugins to support iOS and Android development?
2.3. where can I find the documentation which covers the methods that require to perform the same task on both platforms?

Thanking anyone out their for their time to answer my questions! :)