Install Java JRE 6 (next to JRE 7) on OS X 10.10 Yosemite?

JavaForOSX2014-001.dmg contains Java for Mac OS X (1.6.0_65-b14-462). It will be installed to /System/Library/Java and /System/Library/Frameworks while Oracle Java is located at /Library/Java. No file will be overwritten.
To avoid any trouble (especially with your $JAVA_HOME/$PATH) consider setting up a Mac OS X VM in VMWare, Parallels or VirtualBox with JRE6 only.

As noted by @klanomath "No file will be overwritten": You can have multiple JDK's installed but only one JRE. But each JDK also comes with a JRE.

Apple's Java integration with OS X provides /usr/libexec/java_home that can help you keep the JDK situation straight. That selects among the JDK's and their associated JRE found in /Library/Java/JavaVirtualMachines. See the man java_home page.

However there is no equivalent capability to select among versions of the JRE accessed via /Library/Internet Plug-ins/ http://support.apple.com/en-us/HT202643 provides the only 'official' way to select among those, which requires terminal savvy with sudo and symbolic links. Since you are a developer testing web apps one assumes that will suffice, but not something mere mortals want to mess around with.