We present a density functional study of various polymorphs of graphene fully functionalized with hydrogen, chlorine, fluorine, and hydroxyl groups. We consider the influence of the constant electric field, sheet stretching and compression-induced ripples on relative stabilities of different polymorphs. We found that the effect of the electric field was negligible for all systems. Under mechanical distortions, chlorographene polymorphs decompose, whereas the graphanes, fluorinated and hydroxylated graphenes conserve their identities. Stretching results in moderate decreasing of the energy difference between the common chair-like polymorphs and other structures, but this is insufficient for obtaining the non-common polymorphs. Compression-induced ripples make non-common polymorphs of graphane as well as fluorinated and hydroxylated graphene as thermodynamically stable as the common ones. The ways of experimental observations of non-common polymorphs formed under compression are also discussed.