Nothing is simple in AP, lol !!!
My grand plan for my AP future is to get fast 5 inch refractor with 900-1000 mm focal length. That may or may not happen in 2-3 years time, however all my AP purchases are with that scope in mind. If 50mm guide scope with asi 120mini would still work at 1000mm focal length, than no need for spending extra money. At what focal length OAG is more practical?
At 1000mm focal length, and depending on imaging camera, the 50mm guide scope may not be best (it will guide fine, but it may not be best).
At 1000mm F/L, I would use an OAG (I would use an OAG even at 480mm F/L, but I can understand avoiding the expense and the complexity when it's not needed).
With what you have now, almost everything will work (assuming you mount your guidescope well, as Bob said, and, I might add, even if you don't ).
But as you increase F/L, and especially if you are working with smaller pixel imaging cameras, you will want your guiding to be more accurate, and *that* will be the time to switch to OAG.
As long as you have a guide scope, you need nothing more than the ASI120MM-mini (assuming you don't use a C11 or something as the guidescope ).
Getting an ASI290MM-mini won't benefit the guide scope, but it means that you have a good camera for both a guidescope and an OAG. *However*, whether it's best choice for an OAG will depend on OAG. If you get an OAG with a large prism, then the ASI174MM-mini would be a better (and more expensive) choice. If you get the ubiquitous ZWO OAG, the ASI290MM-mini will be fine.
Why get an OAG with a larger prism? To give you access to more stars for guiding.