SQL Server 2014 SP1 installation iso file question
What files do I need to install SQL 2014 SP1 CU6?


You would need


SQL Server 2014 RTM setup
SQL Server 2014 SP1 Setup. This can be downloaded from here
You would need SQL Server 2014 Sp1 CU6 setup. This can be downloaded from here.


Note: Quite a few times you would find SP1 embedded in SQL Server 2014 installation files in that case you dont need to download Sp1 you just need to download CU6. When SP1 is embedded it wuld be mentioned in product software like SQL Server 2014 Sp1 Developer edition. If you have option go for this one.


  Is there a slipstream SQL 2014 SP1 ISO file available for download or do I install SQL 2014 RTM then SQL 2014 SP1 and then SP1 CU6.Which RTM CU6 file should be used?


There is no Slipsteam file available for download its a way to install updates with SQL Server. Yes you can install SQL Server 2014, SP1 and Sp1 CU6 all together 

Microsoft blogs already has information about Create a merged (slipstream) drop containing SQL Server 2008, Server Pack 1 and a Cumulative Update (CU) based on Server Pack 1.

I also suggest you read SQL Server 2008 Slipstreaming FAQ

Boris Hristov has also written good article see This link. This uses command line to install SQL Server and the updates.

Here is what we should do to slipstream SQL Server 2012 and 2014:


Download SQL Server 2012 or SQL Server 2014 media
Download all the needed SPs, CUs and Hotfixes that you want to
“embed”
Put all of the .exe files in one directory
Launch the setup.exe from CMD by issuing the command:

Setup.exe /Action=Install /UpdateEnabled=TRUE /UpdateSource=”path_to_the_directroy_where_the_hotfixes_are”


Place all the Sp and CU on one location after extraction and give the location in /UpdateSource

For example if location is C:\SQLServerFixes the command would be

Setup.exe /Action=Install /UpdateEnabled=TRUE /UpdateSource=”C:\SQLServerFixes” 



  Is there a separate download required for SSMS 2014 etc.


No, while installing the update you can select the features you want to update.

EDIT:


  Do I have to download a different file for developer edition Or download EE and provide developer license information. 


Developer edition is same as enterprise in functionality but you can ONLY use developer edition for development. Starting from 31 March 2016 Dev edition is free to download for Visual studio DEV essential members. So if you are grab it. Their is no point in going for EE edition.


  SQL 2014 RTM download version without CU as SP1 would cover it? What filename / size would this be - if this information is easily available.


You have to just download as I have said in starting