If ssh starts going down that road, another dozen or so "special cases" later it'll be a horrible mess of magic interdependencies, with determining whether negotiation _can_ succeed bordering on NP-hard.SSH is already down that road. GSSAPI key exchange requires ignoring the negotiated host key algorithm.
No, it most certainly does not. GSS keyex depends on neither public key signature nor encryption, and so can be used with host key algorithms which provide neither, such as "null". It does not _require_ use of such an algorithm; it works with any host key algorithm and in fact has the side effect of authenticating whatever host key is used. It certainly does not require ignoring the results of host key algorithm negotiation and using some other host key instead.
-- Jeff