Hi Taipanfc
My quick reply to your original question/problem would be;
If you have enough entrants to make a separate fleet start go with it. Otherwise, it's a case of handicap racing using the TEXEL rating system as you suggest. If a Viper arrives without a measurement certificate then I think they should have the correct F16 rating depending on the number of crew.
However, if a Viper does turn up with a correct measurement certificate stating their TEXEL rating then they have a choice, either sail as an F16 if there is a fleet or in the handicap class.
This problem does arrive in the UK occasionally at Open Meetings and who's sailing single handed, two up or (F16), usually the sailors sort it out well before racing commences so we all have a good time.
I hope this helps and you have a great Regatta.