You can use ID90 tickets on KLM. Not on EasyJet
AFAIK. Don't know about Croatian.
You need to buy the ticket through your staff travel dept & then just turn up at checkin. Seats are allocated on a standby basis. You can list yourself for a particular flight if you know in advance & you may get some idea of availability. The rules really depend on the arrangement your company has. For us (BA) the ticket routing cannot be changed and it is valid for the named airline and/or any OneWorld airline.
I ID90 quite often on KLM & so far have not had any problems.