Okay so for custom things, to store them you should make a folder containing your items that are int/number values. Somewhere else in the player you could also store what kind of backpack the player has. You can either use an IntValue to store the backpack ID, or a string value for the name of the backpack.
To limit the amount of things a player can have, in your collection script that adds to how much of a thing the player has, you could check in there for what kind of backpack the player has and then allow/disallow the addition.
Pseudocode for collection:
local max_Storage = 5 -- default limit
if PlayerBackpackType == "Big" then
max_Storage = 12
elseif PlayerBackpackType == "Super" then
max_Storage = 25
if RedCandy+amount > max_Storage then
print("Not enough space!")
RedCandy += amount
The code above is as basic as it can be, but should work in theory!