Hi Flute Pastil
A B foot is more or less essential for Orchestral repertoire -plenty of examples in Richard Strauss, Schostakovitch, Tchaikovsky and especially Mahler . Most of my professional colleagues have a B foot which they use as and when required, preferring the response of the instrument without it.
when I bought my new flute some years back, I had previously played on a B foot instrument but was appalled to discover it was an extra £1000 on the new instrument ! so I got the C foot version.
Pros: Can play low B! , better tuning on top C (add RH 2 3 and the b key)
Cons: Heavier to hold , Balance is altered , Sound is slightly darker( I see this as an advantage!)
On the subject of playing in a ballet orchestra, I wouldn't limit yourself too much , everyone I know who plays in one came up through freelancing in all kinds of orchestras- Opera, West end type shows, Standard concert repertoire, Chamber music , Contemporary Music- in fact you are unlikely to develop the skills needed unless a wide range of music has been explored.
On your other questions -no change in fingering required, top note remains the same, The ideal solution is a C foot instrument with a separate B foot if you can afford it. Very best of luck