The Generalized Relativistic Effective Core Potential (GRECP) method is described, which allows to simulate Breit interaction and finite nuclear models by an economic way with high accuracy. The corresponding GRECPs for the uranium, plutonium, eka-mercury (E112), eka-thallium (E113) and eka-lead (E114) atoms are generated. The accuracy of these GRECPs and of the RECPs of other groups is estimated in atomic numerical SCF calculations with Coulomb two-electron interactions and point nucleus as compared to the corresponding all-electron Hartree-Fock-Dirac- Breit calculations with the Fermi nuclear charge distribution. Different nuclear models and contributions of the Breit interaction between different shells are studied employing all-electron four-component methods.