Prob not nute burn then if your only feeding formulex, but could be nute lock out if you didn't rinse your pebbles in ph5.5-5.8 water before planting. Is your hps too close maybe? Its hard to tell exactly what the prob is but if you flush n keep yr ph to around 5.8ish it should help.
That...
They look seriously nute burned to me h20. What have you been feeding them? I'd flush them with lukewarm water n be careful with your nutes. DONT get rid of your hps its perfect for flowering. Fine for vegging too but if you wanna add cfl's go for it.
Lowryders stay small but should still...
Sorry, it just can't be done. The only autoflowering stains are lowryder or lowryder crosses and have to be grown from seed. I know someone who is leaving a male in with his females just so he can get enough seeds for a sog :mrgreen:
You can grow with a mh/hps in a 3ft space if you supercrop your plants. You can use screen of green, or tie them down to stop them reaching height. By doing this you will use up more width than height. You will still need good air circulation to avoid burning yr plants and to keep temps down...