The smaller the compartments, the better well performance.  The key for AICV completions to be successful is to isolate high permebility intervals from low permeability intervals and high water saturation from high oil saturation.  The AICVs will manage the un-wanted water and gas accordingly within these compartments.  For that reason there is no exact recommended compartment length.

Yes, you can run an AICV completion without an inner string (as in most cases the innerstring / washpipe is not required).  The majority of the AICV completions deployed globally have been deployed in this manner and the operational deployments have been achieved with success.  If well control is an issue if heavy losses are expected, in which case the PAV-AICV joints can be utilised instead for a fully closed system for such well control requirements.

Yes, as long as it is possible to provide good zonal isolation.  InflowControl have performed dozens of retrofits, into both cemented liners and open hole (barefoot) wells.