I grew my lemon grass a diferent way. I bought a packet of 'stalks' from a supermarket. Stuck them in a jar of water, changing the water regularly. When they had got a good root system, I potted them up.
1. I've found they seem to like to just sit of the top of the compost.
2. I've never bothered to feed them, they can get quite large, but I would imagine a high nitrogen feed would be best.
I must say I think you've done really well to get your plants from seed. I tried that way and got nothing from two packets.
I only found the above method by accident. It was advice in a cookbook to keep lemongrass fresh, putting it in water like flowers!!
valmarg