you can check http://www.autogari.ro/index.php?lang=UK have fun razvan

There probably is none. If there are buses that take you from Bucharest to Ruse they will be Minibuses that mostly leave from close to North Train station (Gara de Nord). If there is nothing there, Minibuses drivers will direct you to the right place. Your best bet is to just take the train, you can call CFR for the departure times(or see if you can get them on their website)

There is a bus terminal in the southern part of Bucharest, not too far from the state victim cemetary. You should ask there or just ask at your hotel if they have any info.