On the plugs you're asking about I often do add split
rings. Both of the lures aren't really prone to fouling even though the hooks hang a little lower with the addition of the split rings.
That said Beachmaster himself would frown upon me putting split rings on the Atom Jr. He recommends the old Mustads (which are pretty rare) or cut VMC's on all his plugs.
I generally hang a pair of 3/0's off my AJ's, but I'm sure many would feel this is one step too heavy. I know several guys that go with a pair of 2/0's. I think a 5/0 VMC siwash would be the right hook on the back for either sized belly hook.
Depending on what you're doing with that Deadly, a 1 or 1/0 VMC 9626PS treble would be my choice.