In flex abis mode we need not to configure IDLE Time slot unless the the data requirement is high in any particular site because the site will automatically take the remaining timeslots after OML,RSL,BCCH and SDCCH.
Yes add 2nd e1 on the site and increase the no. of static pd channels on the cell, keep 4 static pd channels on most utilized cells and check the dsp usage when increasing pd channels so that a single dsp may get overloaded.
You don’t need to configure IDLE TS in Flex Abis Mode since all the Abis pool is available for both CS and PS services. Any Abis TS can dynamically be configured as an Idle TS. To check this, use MML command to query the Idle TS and you will get no results for a site working in a Flex Abis mode.