Many details still sketchy with the best information from following the maps of various vehicles on BusTimes.
Powys Council now has a website listing the new services - but the links are to Traveline Cymru which is pretty useless. Some link to new timetables (T4, X75), some old ones (eg 74), some just an error (eg X76, X77).
While the new timetables are not generally on BusTimes it is possible to follow where individual vehicles have been from which some timetable info can be deduced.
Celtic Travel have two vehicles on the T4, one of which is doing the Merthyr-Brecon shorts. Williams Travel have two vehicles on the T4 doing full length journeys.
Williams Travel appear to be running five 75-plate vehicles, two B8RLE Evora's on the T4 and three E200s on the 43/X43. Impressive for 1 September!
Tanat Valley have a variety of vehicles out on the X76 and X77 including B8RLEs, a Metrocity and E200s.
Owens Travelmaster have an 08-plate Solo running on the 87 from Welshpool. There is no link to the supposed new timetable anywhere that I can find, but the Solo does appear to be running to this improved timetable rather than the old one.