I think you can put them where you want, but there are constraints to think about and layouts proven through history.

Be sure you are leaving enough wood in the leg. Too many mortises or too big or too close together and you can end up with a leg that is weak at the place it needs to be most strong. Think about how big the parts of the leg you are leaving are. Top two sketches look ok to me on that.

I wouldn’t count on a wall between two mortises less than 1/4 inch thick surviving my mortise chiseling. Thinner and I’m too likely to blow through. Also little strength in that thin wall. Bottom sketch may have a problem with this.

You want most of the width of your rail to go into the leg so it resists twisting and wracking better, but it doesnt necessarily all have to go full depth. The bottom sketch seems weaker in this way. The ‘normal’ way is more like your top two sketches. In the bottom sketch the rail who’s face is toward us is only joined to the leg at its bottom edge, so it is only as strong as that tenon against the leg being bent toward the center of the rail or twisted around the axis of the rail (the twist bit probably isnt’ super important, but bending the leg toward the middle is).

Shoulders on the rail hide the mortis cut of course, so you usually want them at least on show faces.

The traditional rules of thumb would say make your mortis width at most 1/3 of the leg, 5/8ths of an inch in a 2 inch leg. I don’t have a 5/8ths chisel so I’d go 1/2. I’d leave a shoulder at the bottom of 1/8th to hide the joint and at least 1/2 at the top (maybe a bit more for a dining table?) so the leg doesn’t split super easily at the top and leave a ‘horn’ of extra leg at the top until after the mortises are cut and the tenons fit (chopping mortises and fitting the tenon are very risky for splitting that thin end). Where the mortis and tenon go in the leg and rail would depend on how I wanted the rail face to align with the leg face. On a dining table I think I would keep the mortis at least 1/2 inch from the leg face, for strength.

Highly reccomend taking some scraps or overlong parts and doing a test joint. My legs had enough length for me to cut one mortis off on each one and my first attempt was indeed pretty shabby (I ended up not using the first or second attempts, though I would have used the second rather than buy another leg).