Ahh, a question pointed at me with a 12-13" Sailfin Pleco.
Short answer, absolutely not to the washers. A slightly deeper bed (say 4") will work, but can cause problems of it's own. so why not make the gravel less flat? You can make it 4" at the back and bring it down to 2-3" at the front. Also pick your plants wisely. ones with good root structures, swords and stems are the best bet. You can also tie down Java ferns and Anubias which are good options. (just tie them to wood or rocks)
The washer thought will have the plants resting on the surface with the washers attached, or worse, it will snap the stem like a twig.. Better to have them floating then broken..
a bit. and good luck.
120 Gal SW, 150lbs Prem. Fiji, 23Gal Sump, 2x250w 15K MH, 4x65w Actinic
3 x Halichoeres Chrysus (1 surfed), 3 x Nemateleotris magnifica, 1 x Centropyge bicolor, 11 x Scarlet Hermits, 6 x Zebra Hermits, 40 x Astrea Snails, 6 x Nassarius Snails, 3 x Cerith Snails