IPv6 static addressing

Hi all,

Apologies if this is a bit off-topic, but I know there's lots of IPv6
knowledge here :-)

If I have an IPv6 ULA block (or several) using SLAAC, and I want to
additionally give out some static addresses in one or more of those
blocks, am I right in thinking I can pick anything I like as long as it
doesn't have the magic fffe in the middle?

Is there a more formal approach? Is this (SLAAC plus statics where
required) the normal way of doing things?

Thanks,
Richard