

These plates are designed for use with 1/2" tubing. 3/8" tubing, which has a different diameter, would not fit properly.
Yes you can use those plates with 3/8 tubing. The tubing should be in maximum contact with the heat transfer plate to work most efficiently and so you could fur out the plate with a piece of metal ducting tape before you attach it under the subfloor. You should also realize that the heating and cooling of the tubing causes expansion and contraction and the far and near edges of the plate will gouge the tubing over time so some chalk or like material can be applied to the underside of the plate to allow for tubing slippage.
No, these plates are designed for 1/2" PEX (which does have a 5/8" outside diameter).
The aluminum is paper thin - less than 1/16".