Which macs are compatible with OS X 10.11 El Capitan?

If you care to read the tea leaves, the initial OS X v10.11 Developer Beta supports the following Macs:

• iMac (Mid 2007 or newer)
• MacBook Air (Late 2008 or newer)
• MacBook (Late 2008 Aluminum, or Early 2009 or newer)
• Mac mini (Early 2009 or newer)
• MacBook Pro (Mid/Late 2007 or newer)
• Mac Pro (Early 2008 or newer)
• Xserve (Early 2009)

Source: OS X v10.11 Developer Beta 1 Release Notes.

No official requirements are published by Apple.

About your last question, no. There are no macs supported by Yosemite that are not supported by El Capitan.

It's worth noting that every Mac that supports Mountain Lion (10.8) also supports Mavericks (10.9), Yosemite (10.10), and El Capitan Developer Beta 1 (10.11). System requirements in El Capitan are unlikely to change from those in the developer beta.

However, some features might be unavailable on earlier systems. There is informed speculation that Metal, for example, might only work with newer GPUs.