There are no trains from Beograd to Skopje or from Beograd to Sofia. You need to take a bus.

Do the buses cost extra? Thank you 

Also is there a way to just skip out a few stops and go from Belgrade to Bucharest? 

Buses are not included with a rail pass here.

It's possible to go to Niš by train (4 trains daily). 

It's also possible to go into Hungary (with a short bus ride in between that is included). But just to the border station; from there you'll need a bus, too. 

No trains to Bucureşti from Beograd. 

1. Interrail is bad value for money to travel in these countries.
2. The rail planner app or the Interrail website are not the best place to look for reliable timetables. The best is to check the national websites and use the DB planner or DB Navigator app for international planning.
3. A number of railway companies, such as Serbian and North-Macedonian railways, haven't sent their timetable data to the European Timetable Centre, so nobody outside those countries knows about them.
4. There are currently no trains from Serbia to North-Macedonia. You can get from Belgrade to Niš by train but you'll need a bus to get to Skopje (and that bus would probably have come from Belgrade).
5. There are no trains from Skopje to Sofia.

I follow the suggestion of @rvdborgt  due the limited services these Countries are currently not good for Interrail Have in mind that train tickets are quite cheap and often even cheaper as the value of a InterrailTravelday

There IS also what is called a Balkan Flexi pass-which works same way as IR-but only in thsi area (also in TR), but the probs with NO trains overborder are not solved by it. You can buy that at big stations in the countries participating. BUT in RO it is NOT valid on the main CFR-only on a few local trains.

In short: forget any pass, use bus as main means and if you are one of those tipical Brits who ´love the rails so much´ do that only on the few lines that still have a kind of acceptable service (f.e. that there is a train Bg-N does not mean it is comfortable, nice or even fast-that is why the bus wins). Just buy locally-you will be astonished as to how cheap it is-and there may even also be a junior/.student discount you never ever heard about.

Oh-as it seems you are again collecting capitals: Skip Skopje (though you have to pass it anyway): go to the lake of Ohrid, a bit further on. IF you feel really adventurous and are not that easy to frighten: make a detour via KOSovo between Bgr and Maced/Skop.

IMHO Brasov is probably more worth of stopover Buc-Bp as Cluj. Or Sibiu.

Hello, I was reading your conversation as we are planning to do a trip through the Balkan region. Now I’m wondering if there is a train or a bus included in the interrail global pass which goes to Ohrid. We are travelling in Septembre and we’re passing through Skopje.

Thanks for your help!