you need to edit the retroarch.cfg file for each emulator you wish to use

files are located in /opt/retropie/configs/   directory with an aditional directory for each emulator

for example snes is     /opt/retropie/configs/snes     directory and the file is retroarch.cfg

arcade is                   /opt/retropie/configs/arcade     directory and the file is retroarch.cfg

no matter the directory or folder if you prefer, the file you need to edit will be named  retroarch.cfg

now assuming you have the mini pac in keybard mode  the retroarch.cfg should be edited to look like this

input_player1_a = ctrl

input_player1_b = alt

input_player1_y = shift

input_player1_x = space

input_player1_start = num1

input_player1_select = num5

input_player1_l = z

input_player1_r = x

input_player1_left = right

input_player1_right = left

input_player1_up = down

input_player1_down = up
input_player2_a = a

input_player2_b = s

input_player2_y = q

input_player2_x = w

input_player2_start = 2

input_player2_select = 6

input_player2_l = i

input_player2_r = k

input_player2_left = g

input_player2_right = d

input_player2_up = f

input_player2_down = r
which will give you  2 player controls but should work fine fir one player if thats all you have wired up ,this should also work the same if using an ipac2 in keyboard mode
side note if you have already altered the minipac settings from factory defaults you will need to return the settings to there original state
there are lots of ways to access the cfg files you need to edit
winscp you might need root access
or drop to a command prompt  from within the pie ( you need a keyboard connected to the pie)
type cd /opt/retropie/configs/snes
replace snes with whichever emulator you are editing
Then create a backup copy of the retroarch.cfg file:
type  cp retroarch.cfg retroarch.cfg.bak
Then open retroarch.cfg file for editing:
nano retroarch.cfg

So essentially a 9 button control panel  with joystick

exit button (non essential ) exit of libretro based emulators is achieved by pressing 2 buttons at the same time usually start and select  (non libreto emulators have to be set up from within each emulator separately and in some cases cant be changed) however if you want a one button exit then alter the harness so that this button is wired to both start and select leads triggering both to be pressed simultaneously

Start button  what your calling start/select  in arcade is player one start and on other emulators such as nes or snes is just start

select button what your calling coin  in arcade is just that Coin but in other emulators such as nes or snes is select ( if its an arcade cabinet your setting up and want a functioning coin slot then alter the harness to wire the coin slot and the select button to the same lead thus both will trigger the same input)

then 6 game buttons

ES Auto config or controller setup will do just that  however once you set up the controls based on mame but the button A is in the upper righthand corner and you want it to be the lower lefthand corner then edit the retroarch.cfg for snes  to remap the buttons to your desired locations

basically do the initial ES controller setup positioning the buttons how you want them for mame then edit the retroarch.cfg for each emulator that isn't set up quite right  for what you want

ES should see the minipac as a Keyboard

as for analog joysticks i just skip those when setting up just press and hold down any button till it jumps down to the next entry in the controller setup

and when it gets to hotkey hit select button (what you've been thinking of as coin)  once setup you press the start button and the button setup as hotkey to exit out of a game. in this case you press both start and select and the game exits as i said before if you want a single button exit then wire that button to both the start and select leads so it triggers both keys when pressed

one extra note

Group J1

yellow wire 1A is pause ( as far as I know it only works for mame the other emulators usually pause game with the start button) its keyboard key press is p

Wiring and config info I gave was for the minipac not the gpio hat, I don't have the info on the wiring harness or default keymap for the gpio hat that comes with the monster kit.

Not sure which interface you were going to use, either way if you run into trouble let me know I'll try to help figure it out.

As I understand it you will need to install a driver or script into the image for the hat,unless the image has one pre installed. After that i am led to believe that buttons may be pre configured requiring only the correct wiring and physical placement , of course i may be wrong in those beliefs Ive never used a pi hat before. Either way let me know ,I'm interested in how it works out was looking at aPi2jamma from arcade forge but decided to cheap out and get a zerodelay encoder and a vga to cga converter board from amazon for about 1/3 of the cost