The draft does not assure that existing usages are compatible
with each other.
Still, the draft may assure new usages compatible with each other.
However, people who want to have new (sub)types for the new usages
should better simply request new RRTYPEs.
If we need subtypes because 16bit RRTYPE space is not enough
(I don't think so), the issue should be addressed by itself
by introducing a new RRTYPE (some considerations on subtype
dependent caching may be helpful), not TXT, which can assure
compatibilities between subtypes.
For the existing usages, some informational RFC, describing
compatibilities (or lack of them) between the existing usages,
might help.
Masataka Ohta