Relation between socirty formation and Sale Deed

I have purchased a 1 BHK in Pune in 2007 without the parking. The builder did not form the society and after a long wait, society members got together to form the society. While we had the first hearing the builder informed that since his parking is not yet sold (for my flat) he would not allow to form the society. Since I needed the parking anyways I purchased the same and got an allotment letter for the same. He also agreed to do the sale deed with me. However the society members are asking me not to do so since it will spoil all the efforts we are putting in for society formation. They say if I do the sale deed we would not be able to do a conveyance deed to transfer ownership of land and the members would be at loss if there is an future increase in FSI. I have the following doubts: Is an allotment letter a valid document for Parking? Will the Sale deed affect the conveyance deed? Will there be a problem for transfer ownership of land? Is the concept of increase in FSI and loss to society members true?