The metal parameters can be used in force field calculations and will yield good qualitative results. The determination of charges for these atom types is a separate question which is not easily addressed. The parameter tables used for each type of charge calculation are shown there as well. The positive and negative periodicities reflect the minimum energy conformation of the compound. Positive periodicities indicate that the minimum energy conformation is staggered.
Negative periodicities indicate that the minimum is eclipsed.
Refer to S.J. Weiner et al, J. Amer. You are not doing anything wrong. However, the expression is too long for the UIMS2 buffer. There is no definite answer as to what dielectric constant you should use for your simulations. The following guidelines may help you decide.