With small rotary tables and small components you have to accept that the standard clamp kits are generally too bulky or too cumbersome for the job. So you will be making your own clamps as required anyway. Table slots and the centre hole are relatively large too so it can be tricky to find enough table surface in the right place to actually get small parts held down properly without distortion. Best to accept that you will need at least one sub-plate with holes registers and fittings as required.
If you are going to have a sub-plate the number of slots isn't that important. Better to consider other things first. With a horizontal / vertical table take a serious look at how you could mount collets, particularily if you dont have a spindexer. Its a little harder to get at collet held jobs with a rotary table in vertical mode than with a spindexer but not too much of a pain with smaller tables.
If you have a small vice with machined all over body consider cutting a groove across the middle the right width to be a snug fit on the vice body with suitable clamps to hold it in place. Very handy if you need to hold a component off centre to mill a curve. Especially if you establish datums on the sub-plate and vice so you cn do the offset by direct measurement. Setting up that sort of thing directly on the table from scratch can be a total pain. Especially if you have to do it after centering the rotary table so the spindle is in the way.
Everyone knows about spindle nose adapters to put your lathe faceplate on the rotary table. If you are using sub-plates going the other way can also be useful most especially when the lathe faceplate slot arrangements don't suit the job.
Aluminium is fine for sub plates. For sacrificial ones used when you have no option but to cut into the surface as well as the job pretty much anything adequately flat can be pushed into service. I have a stash of ex-kitchen cabinet MDF sheets which do just fine for thin and sheet metal components. Power driver, woodscrews and penny washers do fine for most work holding.
Clive.