"form new subassembly" command question

The answer is NO. SW moves selected parts into created sub-assembly.

You can Save-As a new assy, then delete the parts not needed.

You want to create a sub-assembly in the main assembly, but leave the Parts stand-alone in the main assembly in addition to the sub-assembly?  So you'd have duplicate components stacked on top of each other?  If that's right, why would you want that?

This is doable. You can either create a virtual sub in the main. Or you can create a sub assembly and insert it into your main. In the tree drag and drop your parts you want into the sub you have created. You will need to re-establish mates, depending on how it was designed will dictate how the mates are going to be needed. We do this on a pretty regular basis, the main thing with it is you take your sub assembly and make sure there are no mates defining it in the assembly prior to moving the parts into it. Once this is done you want this assembly to have only 3 of the existing mates relative to your main assembly. All other mates should be relative in your new assembly. Then you have to open your new assembly and establish 3 main mates relative to the 3 main planes and origin point. Now your sub will be constrained within itself and within your main assembly. The first time I did this it took a little while to see how this worked and getting it right so everything stays constrained is the trick. If your caster bolts and plates have a lot of relative mates to the rest of your assembly you will want to look at what you are mating to and why. The main goal to derive a new sub assembly is that most of the parts in it are mated to each other with as few external mates as possible. Sorry if this was confusing I a mused to showing in Solidworks how to do this and not writing it out.

You can dissolve a newly created sub-assembly later on.