Those are some nice fish. Labidochromis Caereulus and Demasoni is a recognised mix, and the colours look really spectacular especially in large groups of each, which your tank will certainly support.
One thing you should be aware of is that the Hongis are also a Labidochromis genus. There is a chance, although it is not necessarily statistically probable, that they will inter-breed. What mitigates against that possibility is that they don't look alike. What mitigates in favour is that they are the same genus. You can further reduce the possibility by keeping correct ratios of male to female in each grouping (i.e. 1 male to every 3 or 4 females) which makes it more probably that they will stick to their own.
Equally, some people will not be particularly concerned at the possibility of cross breeding anyway.
I would stress that plenty of people appear to have kept yellow labs and lab hongi and not had cross-breeding - again, keeping correct ratios of
male/females in larger groups helps - and it is probably more likely that a yellow lab will inter-breed with for instance Metriaclimia Estherae 'Red Zebra' do to similar colourations, so the mantra of 'don't have two species of the same genus in the tank if you want to avoid hybrids' is by no means a rule to live by. Personally I would be fine with yellow labs and lab hongi together.
To fill out your stocking, I would suggest a good group of some synodontis catfish - multipunctatus and petricola are both great. These also help with fry control - i.e. you would not put them in a breeder set-up - but bearing in mind that Malawis kept in good condition are like rabbits on viagra it can be desirable to keep your numbers under control in a show tank.
If you want another group of fish to consider for your tank, take a look at iodotropheus sperengae ('rusty cichlids'). If you can get good stock they are very pretty when mature, and they are also amongst the least aggressive malawi species and so in a group will do will in a tank with labs.
Finally as regards Demasoni, they are very aggressive little buggers particularly against conspecifics, so as a general rule you don't stock other blue fish with them (they are very hard on e.g. Pseudotropheus Socolofi for instance) and you keep a group of ideally 12 or more being very careful to have enough females in the group. Sexing them is hard - I would suggest either buying known males/females in a ratio of at least 1 male to 5 females, plus or minus, or starting out with a group of say 25 juveniles in your tank, sexing when possible, and removing as necessary so as to arrive at that kind of ratio in a group of 15 fish long term in the tank. This does mean you need to set up a return policy with your LFS or have another outlet for the spare males/females (the LFS should be happy to take them back for store credit).
I think in the long term, given great filtration and maintenance, something like this would be nice :-
15 Demasoni as per above
6 - 8 Yellow Labs (2 male the rest female)
6 - 8 Hongis (same ratio)
6 - 8 Rustys (same ratio)
7 Synodontis (male/female doesn't matter)
That said, some would say, and they might be right, that a malawi tank looks even more spectacular when there are just two or three 'primary colours' in the tank - in which case take out the rustys and increase the labs/hongis accordingly. Some of the nicest tanks I've seen even have just demasoni and labs. Again, basically aim at 15 demasoni and another 15 - 20 (subject to your own preferences on numbers) from the above species in groups of your preferences.
As regards the sump, I've never run one, but I think they are great. I agree with star-rider that you should pitch at the biggest sump you can manage in the cabinet, to get the most value out of it. I would still run a powerful cannister filter with that though. If you find you want to help in collecting waste you can add powerheads to direct flow to your taste - Mbuna are fine in a bit of current.
Finally for now, you can, despite rumours to the contrary, keep live plants with Mbuna. Java Fern, Crinium (onion), Anubias, should all be fine and do not require significant effort to flourish in the aquarium (I've done all of the above with africans and I most certainly do not have green fingers). Live plants in my view add a lot visually to a tank - not to say it should be predominantly green or anything - and also give a natural back up to your biofiltration, and assist in oxygenation.
