Is it legal for a potential employer (in Pennsylvania) to have access to or consider a non-traffic summary offense in the hiring decision?

An employer can consider anything it wants in the hiring decision, other than those factors excluded by statute (race, religion, etc.) or contract (for example, if a union contract provides that an employer can only use union labor), and often smaller employers are not even subject to those limitations.  I doubt very much that the information about your offense would be readily available, but it is probably a public record, and therefore the prospective employer could theoretically, if it wanted to go to a lot of trouble, access it.