Hi, glad to see new trackbuilder
Shortly said, RX plugin has some limitations...
- particles - RX uses slot 41 (Cote D'Arboz) to load all RX tracks into this slot. So, it uses particles from France (tarmac). To have gravel particles, simply make a backup of ps_montblanc and replace this with ps_usa or british or australia's particles. But since there is no way to make cameras for RX, you will have not much better smoke efects.
- or - you mean maybe different physics - then use Material Editor found in "BTB/Support/RBR" and attach your custom physics to your textures.
- skidmarks - you can place semi-transparent Walls over your roads - just like found in BTB video tutorials (but you will not have physics from them unless you make them driveable)
- original RBR textures can be found in .rbz files ("../RBR/Maps/"). You can open and extract them with 7zip program easily. All of them are .dds files, so the best idea is to use Photoshop with NVidia DDS plugin to save your choosen textures into sigle files and pack them in your XPacks.
- ditches, potholes - use Road Shape Sections to make them, also using Randomness can be very effective to simulate more rough road surfaces. Refer again to BTB videos how to make them.
Another way - to have more control on your road and roadside shapes - use multimaterial road building method, called eMeL (among other names) which I found very effective also when planning conversions for original (classic) RBR format.
http://www.racedepartment.com/forum/threads/tutorial-emel-method-road-roadside-and-terrain.21890/
As time passed by, I mastered the rules how to use that eMeL method to make conversions fast and non-problemative. However it's not written there, I believe that information from the thread can give you quite big picture how it works.
- you can't make a whole rally from several tracks. All you can do is just to load one track at a while from (RBR game)-Options-Plugins-RBR_RX list.
Happy trackbuilding
m.