Hey I know it's been a few months, but maybe someone else will find this.
For the reduced speed, check out the mod "mini trains lead the way." It's primary function is to make mini trains mandatory by shifting technologies and tweaking recipes, but it also has a setting that nerfs the mini train's speed and acceleration a bit.
For the fuel slots, you can change it yourself pretty easily. Below is a thorough list of instructions for a novice. For experienced users, modify "fuel_inventory_size" in /data/mini-trains.lua.
1: Right click and unzip the mod with the "unzip to mini_trains_2.0.0" option
2: Delete the zipped mod so it doesn't interfere after the change
3: Open the unzipped folder then the "data" folder inside that
4: Open "mini-trains.lua" in your favorite text editor (I recommend Notepad++)
5: Ctrl+F for "fuel_inventory_size"
6: Change the 1 to a 2 (or higher)
7: save the file
8: (optional) rezip the mod and delete the unzipped folder
You're done!