I set up my motors and prop shaft using a solid connecter and then replace with one of the alloy connectors with the rubber middle section. To do this though you obviously need to use solid and flexible connecters of the same length.
Those cheap Huco connecters, often red, aren't the best quality and are really only to be trusted in fairly low-powered, low-speed craft as damage can be done if they break. Raboesch do make better quality ones though, from e.g. Cornwall Model Boats.
I do have to say though, that motor mount looks a little flimsy and alignment may well alter when power is applied! Perhaps a solid connecter is a better bet! Also, you don't look to have a lot of space between the motor and prop shaft to fit a flexible connecter in? Only one that might fit is one of those alloy spring type connecters. Having said that I've just remembered that you can get the stainless Huco type which are pretty short.