FreeBSD recognizes the Ontrack Disk Manager and makes allowances for it. Other disk managers are not supported.
If you just want to use the disk with FreeBSD you don't need a disk manager. Just configure the disk for as much space as the BIOS can deal with (usually 504 megabytes), and FreeBSD should figure out how much space you really have. If you're using an old disk with an MFM controller, you may need to explicitly tell FreeBSD how many cylinders to use.
If you want to use the disk with FreeBSD and another operating system, you may be able to do without a disk manager: just make sure the the FreeBSD boot partition and the slice for the other operating system are in the first 1024 cylinders. If you're reasonably careful, a 20 megabyte boot partition should be plenty.