FreeBSD up to 8.x include ataraid subsystem, supporting software RAIDs. It should support RAID10, but it is quite old and I haven't tested it for some time, including with volumes above 2TB. FreeBSD 9.0 that is RC1 now and should be released in about a month includes new graid software RAID implementation, that definitely supports RAID10 and 2TB disks. Disks above 2TB are not supported by Intel RAID BIOS now at all, but 2TB and below should not be a problem.
Different disk models should not be a problem, but if their size or performance differ, you may get limited to smaller ones.