I want to setup IPv6 in my LAN, but also want to have some machine to use IPv4 only. How do I do such setup so that every machine can communicate with each other ?
All machines supports dual-ip stack.
The simple answer is to use dual-stack. Give all devices IPv4 addresses. Devices that support IPv6 will likely already have configured themselves, but will need some help to communicate with each other. Setting up a zeroconf (avahi, bonjour, etc) server will help them find each other by name.
If your IPv6 provider (Tunnel Broker, or ISP) have provided you with a routeable address block setup
You will need a good firewall as IPv6 is not protected from Internet access by a NAT device.