Hi there, if you already have sailing skills, then it should not be a problem to skip the Viper, go directly to the Gecko 156+ with the daggerboard, as this is the progression board right after the first basic lessons on a Viper anyway.
The Gecko 156+ has the advantage that it is a shorter/planing hull, which can also be used with a daggerboard, it is an ideal progression board to learn the basics of planing, gybes etc. Whilst the Viper is ideal for mainly non-planing first basic steps, it will also plane, but is definitely mainly a beginner board.
Generally you would need 10-15knots depending on your skill level to get planing with either board. The Gecko is not as good as the Viper in non-planing, in terms of going upwind, due to the wider outline/shorter length. The Viper is narrower/longer, so will have a better upwind/rail ability. But the Gecko is wider, so also more stable, which is good for beginners to.
Hope that helps, good luck,