Cats (in my experience) want to put down boundaries or establish who the
alpha male is. With us there was no fighting until we took a certain breed
in. Can you imagine an 11 month old fighting with a 6 year old? It may be
the breed. I'd say males and females=oil and water, but we've had only two
for years then the population explotion.

Hi Hayley,

Zeus doesn't hate you, but he is clearly bitterly hurt that you introduced
an interloper.  Until now he has always been top cat.  It is important that
you impress upon him that he is still top cat.  Even though he is top cat,
he is not in charge - you are - and you need to regain control.

In everything you do with the cats, Zeus must be called first, fed first,
dealt with first.  When you put their dinner down, say "This is Zeus's
dinner .. this is Miff's dinner".  Assuming the cats sleep downstairs, say
goodnight to Zeus first, then Miff.

In order that they get to know each other, arrange an assortment of
cardboard boxes in one room, then introduce the cats: Zeus first, Miff
second.  The boxes will allow them a much-needed hiding place.  One cat in
hiding takes the pressure off both cats.  Supervise them so that you can
intervene if things get too rough.  Miff may fluff out her fur and strut
sideways, trying to look bigger than she is.  This will tell you that she
feels intimidated by Zeus, so be prepared to rescue her.  If Zeus attacks
her, physically or verbally, tell him off firmly.  If he is either passive
or more positive toward her, praise him keenly.  You may even wish to have
some treats ready for really good behaviour, or sustained passive behaviour.
Naturally, Zeus must have his treats first.  There's no need to labour this
or tease Miff while he eats; simply offer treats to Zeus with praise (so he
understands why he's being treated), then offer treats to Miff with praise.

Never allow Miff to do something Zeus has never been allowed to do - cats
don't have green eyes for nothing!  :-).

Don't allow Zeus to rule you because he's feeling hurt.  Stand up to him,
but let him know he's wanted.  Then walk away and let him work it out for
himself.

It is going to take some time for them to get used to each other, so it's
good you're able to separate them in different rooms.  I hope you intend to
have both cats de-sexed.  Apart from inproved health, this will also ease
their aggression.

For now, just keep plugging away at the top cat, second cat rule and keep up
with scolding bad behaviour and praising good behaviour.  Things will start
to improve sooner than you think.

Good luck,
Spider

Hi, Hayley.  I notice you haven't gotten many responses, so I'll offer
mine:  

Continue to keep them apart when you're out of the flat.  Put a towel
with Miff's scent (rubbed on her fur) in his room so that he gets used
to that.  Spend time alone with each cat in it's separate room when you
first get home; then let them come out together for as long as you're
there to supervise.  Realize that kittens are supposed to "play fight".  
Do get some fun, interactive cat toys to help focus some of Zeus' energy
elsewhere.  But insofar as no one gets hurt, let them work it out.

As well as having patience and serving as referee, have Zeus neutered
absolutely as soon as your vet is willing to do it.  Research and
practice both indicate that this can and should be done quite early.  
Same for having Miff spayed.

Hth.