Wait: if this is all about testing OS X browsers, why do you need OS X Server at all?
As for rack options: a Mini is 2.0 x 6.5 x 6.5 inches according to everymac, which makes them inconveniently a smidge taller than 1U (1.75"), and inconveniently too wide to fit 3 across.
However, if you can mount them on their sides, you can probably fit 8 across in 4U and still have a bit of space between them for cooling. Heck, remove the lid and optical drive, that should help.