If I'm understanding you correctly then the easiest way to have the mini joystick control the slew is map it in-game under axis commands menu for whichever aircraft you intend to use, there is a horizontal and vertical slew option that you can't miss as long as the aircraft utilizes it. Make sure to not have the devices programming software mapped to anything on that input. I'm not sure if it defaults to mouse x & y but if so then just change it to the fall back option and activate the software program afterwards. I don't believe it is necessary to have a profile activated to still map in-game controls but I use the software to program the slider switch on the throttle so I always activate a profile through the devices software but since I have all the other inputs in their fall back state I have no problems mapping them to whatever controls I choose within the game itself. Here's my device profile along with the print-out that I have included the button name/number of each control for the device. Notice that I also remove Mode 3(S1) from within the software, it's not necessary but just wanted to point it out incase you were wondering.
The other way to map either mini joystick to the slew is within the device software by changing both axes to (Mini-X & Mini-Y) to bands and assigning the proper keyboard controls to the proper direction you prefer. Make sure you have three bands per axis so the resting position will be within the middle band not mapped to anything, I would suggest either equal bands widths 33% each or what I typically do and map my middle band (deadzone) to 50% and the two outer bands to 25% each but it's whatever you are more comfortable with. Sorry for the rambling but Hope this helps ya.
Let me know if you have any troubles opening either doc. It wouldn't let me upload the *.pro doc so I converted it to a .txt file, should be a simple conversion back for you. ST Profile_ DCS World X56 (F-15C).pdf DCS World X56 (F-15C).txt