Can I mount other ufs disks, say from BSDi/FreeBSD, and vice versa?
Maybe. First, although Solaris, BSDi, FreeBSD, and NetBSD share a common-heritage file system, the Berkeley-style ufs, Solaris has made extensions. The 32-bit UID field has been modified in Solaris to be a pointer to a parallel “Shadow inode” with Solaris ACL information. Also, the superblock has an additional inode field in Solaris and 2 fields have different byte swappings. Reportedly, you can mount, say, zip disks from FreeBSD, on Solaris by doing a fsck on them before mounting. Fsck makes these fields Solaris- compatible. Your mileage may vary and you should test this (in both directions) before trying this on live data.
*Sadly, we had to bring back ads too. Hopefully more targeted.