Formation question regarding position of center

[volcanohead]

According to the NFL rules, where can the center line up? I know that he can line up where the guard usually does, like in the single wing formation (TE,G,C,G,T,T,TE). But is this legal: (G,C,G,T,T,TE,TE)? Can he line up in the tackle's spot (TE,C,T,G,G,T,TE)? If so, does he need a TE next to him? Would this be legal: (C,T,G,G,T,TE,TE)? Thanks for your insight.