Is a normal partition (eg. ffs) allowed to start from sector 0? I have

yes.  the reason it `works' is that most file systems actually ignore
the first few kb of it's allocated space for bootblock/disklabel reasons.
i think ffs skips 8k and, eg, iso9660 skips 32k.

   many disks with partitions like these at the moment and they seem to
   work just fine. (I hope not by sheer luck.. ;) I know that I can't do
   that on i386 if I want to have a bootable partition, but it seems to
   work on eg. alpha just fine. What about sparc? Installboot man page
   tells that the first-stage bootprogram is written into the superblock,
   so I don't see any reason why it shouldn't work. How does the superblock
   and the disklabel mix?

OK, as i recall you have a disk sd0 that has a partition `b' that starts
at sector 0 and goes for some size.  you wanted to load the miniroot into
this partition so you could install, right?

this fails because sector 0 (well, sector 15 i believe) contains the
*real* disklabel of this disk, so if you attempt to write the miniroot
over there, you will wipe away this information with whatever is in
the first 8k of the miniroot (probably zeros!).
   
   The install document tells me to put partition boundaries at cylinder
   boundaries for SunOS filesystem compatibility. This is only a filesystem
   thingie, ie. I can begin the swap partition wherever I want, right or
   wrong?

this is not so much a `SunOS filesystem compatibility' as it is what the
SPARC hardware needs -- you can only boot from a partition that starts
on a cylinder boundary.