I keep a variety of sizes in all my L tanks and let them decide on which they like. I suppose odd size ones don't look the best and can certainly see an advantage of having all the same size.
I will say that my L134's and L066's I rarely see in the 12 x 5 x 2.5's (in saying that I've never given them the option of just that size). The L201's/L270's seem to use this size the most.
I'd perhaps suggest on maybe looking at a couple of slightly bigger in 14 x 6 x 3.5's or I even use the medium oval shaped for the larger L's and bigger peppermints.
I also have had a lot of success with the rectangular style caves. I'm pretty sure most of mine were Zig Zag brand which are a darker colour to the others but I'm not even sure if you can still get this type anywhere.
Best of luck.