I'd do it again if travelling the same route. So it's up to everyone how to do it, I'm just sharing my own experience. To sum up: it is of course a risk to do it this way, but it can save you some money. But I wouldn't rely on it anyway, it happened to me few times in Romania and Bulgaria that the card machine was not working and I had to pay in cash. I'm not sure if they also accept credit cards to pay tickets, I didn't ask. I had enough BGN after changing it in Bucharest, but there was enough time to find and use ATM in Ruse, so no problem at all.
It was 19 BGN and reservation of seat (which is NOT obligatory in this train, just recommended) was only 0,5 BGN! So I saved cca 10-11 EUR by not buying the whole ticket in Bucharest. As there is 55 minutes gap between the two trains, I had enough time to buy a ticket Ruse-Sofia in local ticket office. After that Bulgarian border control took our passports from the train where we waited for them like 10-15 minutes. We arrived to Ruse with less than 5 minutes delay. On the train's display was written that it's going only to Giurgiu (last Romanian city before border) - don't let this trick you, it's the right train anyway and going to Bulgaria. 2/3 seats were taken and quite a lot of people got out in few stops in Romania. Ron Ruse, Brian Weiss, and Layton Talbert (third row) Louis Walthers, Ron Hoffman, Mark Idler, Brad Bell, Gary Seaman, Glen Wimpe, and Doug Knight. The train INT 1095 had only one car (Friday 24th November, don't know if it's different according to the day of week), but it still wasn't full, max. I asked for the full price Bucharest- Sofia and it would be 155 RON. It was 60 RON with reservation (which is obligatory on Romanian side). As I wrote above, I bought a ticket from Bucharest to Ruse in Bucuresti Nord international ticket office. I think it can be useful for others doing the same sometime later. Hello! I'm back from my trip so I'll share my experience.