Singing in choir is not necessarily incompatible with a solo career, unless you are so much advanced that choir singing could affect your professional reputation. But it could depend on the approach the choir director has, and also the sing professor the singer has.
Yesterday I attended a professional choir concert, performed by the RTVE choir and soloists.
Program: Benjamin Britten and Schubert.
The soloist sang their parts and for choir-only parts, they integrated perfectly in the choir.
It is true that both activities are different, even opposite in some aspects, but controlling those aspects so that the singer fits in each situation could be (i.e is) a good exercise and helps in mastering our own voices.
I say this not for me, as I cannot start a solo career at my age, but because I sing in an amateur choir that faces, nevertheless, important musical challenges. For those special occasions, we get some help from soloists that are already in the professional level and are also singing in professional choirs.
They know how to "cover" their voices and avoid been heard above the other voices and achieving a good blend, a necessary requisite for choir singers. And when it comes to sing as soloist, they know how to project their voices and be heard by the last row of a big hall
I posted some information on the last challenge for my choir, with symphonic orchestra and soloists. In this case, the soloists did not integrate in the choir as the program (two symphonic-choral masses) was being recorded by the TVE second channel (and also, they were not paid for that):
http://www.8notes.com/f/25_221840.asp.
TVE: Radio Television Espaņola