Supplemental Poverty Measure (SPM) variables use a different family unit definition (identified in the variable SPMFAMUNIT) than the standard CPS family definition; the weight SPMWT can be used with these variables to calculate nationally-representative population estimates. Note that you should keep only one person per SPM unit in your analysis (though restricting your analysis using PERNUM=1 will not work in this case due to the fact that some family units do not have a reference person).
In reference to calculating county-level estimates, the CPS is stratified at the state level and is therefore not designed to generate sub-state level estimates, it is primarily designed to calculate reliable national estimates. The CPS is also stratified at the metro area level; the U.S. Census Bureau cautions against estimates for smaller metropolitan areas (though they will mostly be present in the data) but notes that larger metropolitan areas like NYC or LA should generally be fine. In addition, as is noted on the description tab for COUNTY, to preserve respondent confidentiality, not all counties are identifiable in all samples. Despite documentation cautioning against county-level analyses, many people use the data in this way; it may be helpful to consult the relevant literature for any guidance on how other researchers do sub-state analyses. For more discussion on county-level estimates using CPS data, see this forum post.