[nuzuki]

I have 2 lots of 4 chicks, some are 9 weeks today the others are 3 weeks. When is it ok to mix them and how should I go about doing so?

Also ive noticed the second lot of chicks which are mixed variety are ALOT more skittish and do not like being handled. One even pecks me aggressively when I put my hand in and they shreik really loudly when handled. Whereas the first batch of chicks are very placid. The cockerals being alot easier to handle than that hens. Is this just the case with some breeds?

The light sussex tends to be the bully and the maran hens do not like being left alone with the light sussex they want their maran man to keep them safe it seems. The marans all huddle together and the light sussex tends to be an outsider. Have I got racist chickens? 

[Helenaj]

You won't be able to mix the two lots for a good few weeks - at that age gap your older chicks will most likely kill the younger ones without a "mum" to look after them.
As for handling, it varies - it could simply be that you've been a little more hands on with the older birds. Plus chicks are noisy and will squawk at anything, just be gentle when handling so they calm down.

You've not got racist chickens! There will always be a head hen and your Sussex appears to be it.