Yes IMRO (or VMRO as we call it) was a national liberation movement for Macedonia. Vlado, who killed the king was a member, but IMRO was also connected with Ustase at the time.
My guess is that their "politics" for national liberation found some similar points.
It comes at no surprise since Macedonia does not share physical border with Croatia and thus there were never any "tensions" between these two nations.
I'll be honest - I'm not sure what the official Yugoslav interpretation of the king assassination was. But I'll look it up.
As for IMRO, this is mostly true:
http://en.wikipedia.org/wiki/Interna...ost-war_period
There were indeed some fine communists in the IMRO, but purges still happened.