The best way to improve the relations between slaves is to make them have sex with each other during an interaction.
Yes, it's fine to put the Constants mod last.
Viewing post in Aric's Expansion Rebred
No, it's basically self explanatory for in-game use, but here's a basic guide.
To install: Open the game, press Mods in main menu, select Constants in the list(it should probably be last), apply mods, and restart the game.
To use: press Constants in the main menu, change the values however you want, and close the menu to save the changes and return to main menu. The left column is variable names (hover mouse over for description), the right column has values that can be clicked to change. The mod will automatically reload any saved changes whenever you start the game.
Enabling Ralph's Tweaks with Constants is a special case and will require a restart to go into effect.