First off, if your machine shipped with Lion, you cannot install a previous OS onto it. The firmware (hardware) in it is tied to the OS version that it ships with. Many of the hardware components only interact with the newer OS, as the older OS won't have code to deal with them...so you are stuck with Lion.
Second, any time you need to "hack" something to install it, you are taking a risk of it not working. There's a reason it won't install...the application is very old compared to the OS you are trying to install it on. FCP 6 is almost 8 years old...ancient in software terms. The newer OS didn't take into consideration, support for that older version.
This is a common issue everywhere. Many companies stick to older hardware, running older operating systems, so that the software they rely on will work properly. This is why some companies are still on Windows XP...and why many won't update to Windows 8...are sticking to Windows 7.
If you need to run FCP 6, then you need to keep an older computer running an older OS that will support it. Or you need to get more current software. Thus are thing with old software and newer computers and OS versions.
Shane
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