Ok.
So what you want to do is to create your Operations to be pretty vanilla. It looks like you already have.
Then, the Resource Groups do not really matter here. You can have every resource in one RG or multiple. When you create a Capability, you need to enter a default RG, but then can add resources from any RG. So, I’m not sure what the point of the default RG is.
There are 2 things to consider when making the Capabilities. The first one is order of equipment for the system to select. I am going to make some assumptions, so sorry if I am wrong. You say you can Tap, I would create a Capability called Tap, add the Tap operation, and then some RG. Then I would add all of the resources to the capability that can Tap. I imagine you would want your Drill Presses to be the lowest priority (lowest means picked by the system first). Then you may want your old manual Bridgeports as the next priority. Then you would want your mills as the highest priority.
You want to put ALL resources that can do the operation as an available resource. More than likely, you will never use a mill to just tap something, but just in case you actually have that much tapping work, the system will schedule them.
The second thing to consider is size. I work better with brakes, so I am going to use them for my example. You have 3 brakes, one 10 foot, one 6 foot, and one 4 foot. Here is where you would create Capabilities that call out length. You would have your generic bend/form operation with any RG again. But you would create 3 separate capabilities; one 10 foot and under, one 6 foot and under, and one 4 foot and under. The resources for the first one would be the 10 foot brake only (and yes, you want to create capabilities with only one resource as it is easier to just add a new resource if you buy one than trying to convert a resource to a capability). The resources for the next one would be the 6 foot brake as the priority with the 10 foot as secondary. The final one would have all three brakes with the 4 foot as priority, the 6 foot as second, and the 10 foot as last.
Now, using the second type of Capabilities requires your MOMs to be created differently. You cannot just Add Operation and select the Operation. Technically you can, but then you would have to go and change the scheduling resources. Instead, you want to use the Engineering tab to drag and drop your Capability to the tree. This will ensure the correct Capability is chosen as the person needs to consciously select one of the Capabilities with a length that will help the system select the resources based on capacity and load.
Sorry, I tend to ramble, so let me know if you have any questions or if I missed anything you were looking for.