Having this limitations in mind, I started my project addressing one string at the time and I've got plenty of time to display a minute based clock. Since all strings use the same common code and therefore the same variables, I can only differentiate them using different ports.
a 16f690 has 3 ports A,B and C by different ports do you mean different pins ?


Can all the outputs be on the same port, preferably sequential?
ie RC0:4 OR RC3:7 or similar