We have done a test running Spark and Wildfire on an IPv6 network. Yes, we can confirm that there is absolutely no problem running them on both types of networks IP4 and IPv6. The target platform was w2k running on laptops.
Java supports IPv6 as following (single and dual stack options):
1 - Do we have to do something to support IPv6 in WildFire ?
Turn on IPv6 support in your OS.
2 - Does IPv4 and IPv6 clients will be able to connect to the same WildFire server ?
This will be somewhat dependent on what your OS does. Wildfire is just listening on all ports on the default address (0.0.0.0 in IPv4 speak, :: in IPv6 speak) so if your OS has some 4in6 addressing abilities (like Linux) it should just work out of the box. On Linux, when I run a netstat command I see things like this: