For example, path of exile, diablo and anything fitting a multi cell inventory.
Right now, I have slots ordered as "Slot#", each with a corresponding grid placement. For example, 0,1.. 0,2..0,3 or 2,5 etc.
My current problem is being able to use these coordinates to 'hover' other slots correctly.

3 comments
What do you mean exactly? You mean where one item takes up say two slots?
Yes. For example, a helmet would be a 2x2 area, a longsword is 1x3 and a greataxe would be 2x3. This way items must be specifically hoarded instead of collected. A quick google of said games above would provide a better idea.
Pretty complex. I doubt you can find a ROBLOX tutorial on it, you should instead search how other people did it on a different platform and then apply it to ROBLOX.