With regards to save games, we are doing everything possible to ensure compatibility. We have implemented import routines for the Network Extensions Project roads, T++ roads, and T++ road customisations (speeds, lane changes, vehicle restrictions). We are hoping to do similar import for those using SomeRoads mod as well, but not sure if we'll get that for initial release. For those using Improved AI++ mod, that will be included too, but without the settings slider (it can be added if there are sufficient requests).
For those considering using the current T++, I would suggest waiting for the new mod; the current T++ has some performance issues which are mostly resolved in the new mod (and also every feature in the new mod is optional so you can disable any feature that causes fps drop).