Markus,
13.5 would be a little tight. I use 16x20 Cesco-lite trays for 8x20s and have processed up to 12 negatives at a time. But processing 6 at a time is probably optimum. Process the film horizontally and rotate it by pulling out of the top of the tray (the side away from you). 13.5 in that dimension would be awfully tight for 12x20s. Do get at least one 16x20 tray for the developer. For the other trays the 13.5 would be okay.
I process emulsion side up, but emulsion up or down seems to make no difference. There are good arguments for doing it either way. I think one's preference is a matter of habit more than anythng else.
And I use flat bottom trays. I switched to flat bottom after one negative, once, had a big "X" on it from the molded form of the bottom of the tray. To this day I cannot figure out how that happened. Otherwise the negative was fine.
I have found that rotating the film, so it is pulled out by a different side, makes no difference whatsoever in densities on the negatives. For years I rotated the film every few shuffles. I then did some tests, developing film with rotation and then without--always pulling from the same side. No difference in the prints that I could see. So I haven't rotated film in the developer in over ten years. It won't hurt anything to rotate the film, so if that feels better, go right ahead.
But do get a developer tray larger than 13.5 inches.
I use close to 4 liters of developer per batch of negatives. Three liters would be okay, too, but not less.