Threads a bit old but I thought I would add my two cents as I am familiar with both styles. ( Note: I am not using the word proficient.)
In many ways, I felt going from steel string pick style guitar to classical is similar to going from classical to flamenco. Sure there is some overlap and having facility with music theory helps but there are also some pretty major differences between classical and flamenco and it will take a bit of time to train your ears and muscle memory.
Right hand technique is very different. Not just rasqueados, but there are lot of techniques that one doesn't commonly use in traditional classical playing...golpes, alzupua.... I see traditional flamenco as being a very percussive/rhythmic endeavor to accompany singers and dancers. Classical has more tonal subtleties,IMO.( subtlety seems to be something that doesn't work in a loud dancing/singing festive environment. Modern flamenco on the other hand seems to be a different beast and is a bit of a hybrid of styles) Flamenco is a group activity, classical more solo oriented.
The chord voicings and fingerings are usually very different as is the common use of the modes of A Phrygian and E Phrygian. The rhythms ( or compas) of many traditional flamenco styles have a 12 beat pattern. (something like 6/8, followed by 3/4). Most of the classical pieces I play are in 4/4 or 3/4 timing either in major or minor keys. Getting used to the very different rhythm may be one of the most difficult adjustments.
As far as the instruments themselves, I think flamenco guitars were relatively inexpensive working mans guitars at the turn of the century ( I mean two centuries ago
) They were made with local materials and not expensive imported rosewood. They didn't have the sustain that "classical" concert guitarist desired but they did have a sharp attack and were somewhat treble heavy to cut through the other participants. I think guitars with more pronounced/deep bass were developed a little later on and coveted by concert players. A golpeador on a guitar used for flamenco was essential if you didn't want to damage the guitar. ( I'm guessing that didn't help with sustain either)
Playing posture is also taught differently....
All of which makes me wonder...what really are the similarities?