Hello, answering all your questions:
There is no limit to model and IFC file size for BIM Vision in 64bit version (exception hardware limitation),
All IFC2x3 and most of IFC4 files be viewed (ICF4 is in developnent). There is no IFC extensions, all new classes become standard in subsequent IFC releases (we read most of them). If you have any file that is not read correctly, please send it to us.
We do not plan to support other formats than OpenBIM