What is the ruling on benefiting from acquired expertise and existing programs in a previous company to create a new, private program?

There is no harm in benefiting from your experience within the company and your previous experiences. As for using the company's software, there is no harm in that if the company does not object or has given you permission. Otherwise, it is not permissible to infringe upon its rights or benefit from them without its permission. You must seek their permission for what you have done if it was without prior authorization.