Reference Page 37 Driving in Rwanda. The requirement for red triangles and fire extinguishers only applies to large vehicles possibly Pajero or similar and above. I took a Ugandan Toyota over in February 09 and there was no requirement for these items in a car. I know because I was roadside checked en route to Kigali!
Private minibuses (page 65) really have taken over from the matatu type taxis for travel between towns which are realy only used in and around Kigali. Each company (eg Virunga travel for Musanze and Gisenyi) has their own office and bus station. The minibuses are bigger than matatus and comfortable. We did get tickets from Kigali going north for a bus leaving right away.