For metal holding, I’ve had good luck with double-sided carpet tape for small parts. The areas are small enough that the thickness of the tape is uniform enough not to affect the surface plane. I’ve used the duct-tape/super glue/duct-tape method for larger jobs. Non-uniform thickness can be an issue. Another reason for slower cutting speed - Less force requires less work holding. When cutting from a plate I use regular hold-downs in the threaded inserts.
Tabs can be tough. I try to make them as small as possible, particularity height. I think short wide ones are easier to remove. Slower cutting -> easier to hold, smaller tabs. I use a small belt sander, files, and sandpaper to clean up. One mistake I’ve made repeatedly is trying to be cheap and cut as close to the edge of the plate as possible. This doesn’t allow enough meat left in the plate to hold the tab and small parts go flying around.
I have used 1/4" tools for roughing aluminum, but I’m usually doing detail or contour cuts so I use the 1/8". To rough, just use lower DOC, slower cutting speeds, and smaller stepovers. I have found that climb milling works better than conventional milling. I would have thought the opposite because I use conventional milling a lot in non-fibrous wood like MDF. Winston Moy has a good video on aluminum milling with 1/4" mills.
I use the drag knife mostly for architectural models. I have an architect friend who sends me 3D models of buildings he is designing. I import them into Fusion 360, do all the scaling and cleaning up they need, and slice them horizontally. Then each slice (Lots of slices!) is cut out of card stock - the ones from Dollar Tree work great - and stacked up to make a building. Then he uses them to show clients. Apparently most prefer something they can look at and walk around than see on a screen.
The only vinyl cutting I done was with stuff I had laying around. I think it was about 0.020" and both drag knives worked well. It’s a little hard to hold down because it’s so soft. I haven’t looked for any lately, but I heard thin sheet plastics in general are hard to find due to people making face shields and barriers.
Instead of T-Track I’ve been using press-in inserts with 1/4"-20 threads inserted from the back of the board so they can’t pull out with good success. A bag of 100 is about $15 and they’re re-usable. When I face the back of a new spacer I also mill holes and countersinks for the inserts on a 4x4" pattern, along with 1/4" alignment holes to keep all the spacers registered.
I had (have) the same problem of cutting too deep, either because I mis-measured, or the design was wonky, or I don’t get Z zero set exactly right. Now for anything that goes all the way through, like contour cuts, I put a piece of 1/8" insulation foam from Lowe’s under the stock. It’s small cell foam and is already fairly compressed, and I haven’t had any trouble with it deforming and screwing up the cutting plane as I tighten the clamps. If I want to go the the bottom of the stock and it goes a little deeper, no harm done. In fact I frequently design my contour cuts in F360 to go 0.005" below the bottom surface because the bottom edge ends up looking better. It doesn’t work as well for small pieces of stock you’re clamping because you can tighten the clamps unevenly enough that you crush one corner. I use it routinely with aluminum plates.
Wish I lived near a shop that generated aluminum scrap. Shipping costs for that stuff is rough.
Have good one.
Bill