Not sure if the mix woud stratify. When mixing large and small dense seed, the small seeds can settle to the bottom more, leaving an uneven stand.
Oats can be considerably larger than rye and somewhat larger than wheat. A large seed bin can go a long time before coming empty Might not be a bad idea to see if the oats are floating ontop more after your 1/2 way through the seed bin.
The seeds aren't wildly different than the others, but something to keep a better eye out for when mixing smaller seeds, like turnips, vetch, or beans.
Soil depth s a bit different ideally for each type of seed. What you settle on depth would be more dictated on what soil you have. IF you got looser less compacted soil, you're going to want to go deeper. IF you're concerned about water, you might want to go lower. Heavier clay soil, you can go shallower.
Also, are you going no-till with a no-till drill. Trying to use and older drill in no-till fashion. Or, are you doing soil prep like discing.
Rye is about 3/4 inch, wheat is an inch or so, oats are usually done about 1.5 inches. There is a range which depends on what your planting into.
Also, with no-till your settings might be adjusted on how well you cut through, or even better push aside the thatch or last crop residues.