I wouldn't JUST do sumo's. Use various stances/grips. Sumo is designed to lower the strain on the lower back. As far as your "bad" technique, start with just the bar, get the motion correct, then add weight slowly. You'll get it. My friend had terrible technique, I just couldn't "see" what he was doing wrong. Then one day it just clicked, he was on the ball of his feet, instead of being more flat footed. Something as small as that can throw off the whole lift. The best exercise I have found to isolate the hams, have been box squat variations. This week I am not box squatting, but just loading up the weight for regular squats. I do calves and deadlifts on the same day, as well as some clean and press.

Add strength to your dorsi-flexors, tibialis anterior (shin muscles), abs, back, and feet. Get some Nitric-oxide supplements (L-arginine alpha ketogluterate). It has been shown to speed up fast twitch fibers, and make slow twitch fibers "act" like fast twitch fibers.

As far as plyos, honestly, I only do a few as well. But... I play a lot of ball. The ones I do the most are depth jumps, box jumps, water jumps out of chest high water, jumps with feet pointed outward/inward, ankle jumps, lateral jumps, POWER SKIPPING!, & tuck jumps.