Taking a bus in the middle of my interrail trip

Hii!

I’m planning on buying the global 10 days within 2 months pass. I would like to travel from Rijeka (Croatia) to Trieste (Italy) but according to the rail planner app it would take like 5,5 hours by train. Instead, a flixbus would take only 1h 50 mins. So, is it possible that I use an alternative transportation from one country to another in the middle of my trip? Or do I need to use only my Interrail pass? Is this a good way to save one travel day for my pass or will something go wrong? :D

Thanks in advance!