I am also trying to do this - in my case using an existing Dell 2407WFP 1920x1200 IPS LCD in the center flanked by two newly bought cheap 23" 1920x1080 TN panels. This way I only had to buy 2 cheap monitors to achieve eyefinity.
Although I got it to work, it was a hassle (mostly because I couldn't figure out the monitor timings). Because I had to force custom timings by overriding the auto-detected driver, it's difficult to switch back to 1920x1200 for non-gaming work (it requires a reboot and driver reconfig each time), so I basically leave the center panel at 1080. Due to this, my advice is just to buy three monitors of exactly the same size and rez, and be done with it.
On the other hand, ATI may add support to the catalyst driver to allow setting of custom options for scaling and maybe even resolution for each of the individual monitors in the group. Then 'mix and match' of all types would be easy. The eyefinity support currently seems very basic.
If you still want to try it - the only way I know of is by creating a custom monitor.inf driver file for the center monitor, to trick the driver into thinking the bigger monitor only supports the resolution of the other two. The approach is described here, with some tutorial info:
http://www.widescreengamingforum.com/forum/viewtopic.ph...
In my case, I did finally get this to work, where the 2407WFP is running at 1280x1200, but with only 1080 active lines. I just changed 'v active lines' from 1200 to 1080, and 'v blank' from 35 to 155 in the Phoenix EDID designer (see the link), created and installed my own .inf file. This puts a 120 pixel thick horizontal blank bar at the top of the center panel, keeping my display at 1:1 pixels for quality, and the screen at about the same height as the other two, side monitors, for good eyefinity. The pixels are a tiny bit bigger still on the dell, so the center image is slighly taller. No big deal (but again, if you just got 3 identical monitors, they'd line up perfectly).
It almost worked right out of the box - when I use 'extended desktop' without the eyefinity group, all 3 images all look about the same height (the center monitor is running at 1920x1280, native 1:1 pixels, no GPU or monitor scaling - for 'extended desktop', you can tell each monitor whether to do GPU scaling or to stay centered, and you can have them all at different resolutions).
But when I turn on the eyefinity display group, the center image stays at 1920x1080 pixels, but the GPU scales it up to stretch across 1920x1200, which not only makes the center image too tall, but also blurry due to the interpolation of 1080 lines across 1200 pixels. Ugly!