Hm, this is a basic Jira SOAP question AFAICS, but Ok, I'll try.
One of the complaints about the Jira SOAP service has been that not
all information available in Jira can be easily retrieved. Hence the
existence of this add-on to the SOAP service, adding some often/long
requested but not provided methods...
Getting all Issue fields probably requires you to 'merge' the results
of both queries into one set of fields and use that as you need to.
There is not always a simple straight road to a solution, some
crafmanship is in combining the data into something useful.
Jira systemfields all have known names, and these could be compared to
a list you provide in your code, or configure externally.
HTH