It really depends on two things: first of all, what your objective is. Do you want to raise fry, or do you just want them as part of the community? If you want to see breeding behaviour but are accepting of the fact that some or even all of the fry won't survive, a pair can be maintained in a community setting. And second, cichlids are notorious for having varying personalities. You could get a pair of Kribs that are OK with the likes of Corys, and will only do a little chasing when the Corys stupidly amble too close to the fry. Then again, you could end up with a truly psychotic pair that will attack the Corys (and anything else) relentlessly - and, despite their size, they can do some real damage.
If you do want to raise fry, I'd give the pair the entire bottom of the tank to themselves, and keep them with a small-ish number of something that is fast-swimming and top-oriented. Pencilfish are a great (and common) choice for Krib dithers. This type of tank is not going to be very interesting (to most) to look at, but it will most likely result in a bonded pair, minimal losses to the dithers, and a high survival rate for any fry.
If you just want a pair in a community tank, the most important things that you can control are:
- Make sure the bottom isn't too densely stocked.
- Think about territory. Make sure that you break up the lines of sight really well, and that the Kribs have a cave over to one side of the tank so that when they are breeding, they're not trying to claim the whole tank.
- Try to keep them with fish that are robust. Armored catfish can work (but can also be risky if the lines of sight aren't properly broken up and/or if the bottom is too densely stocked. These types of fish will take any eggs or fry that they can and the Kribs will punish them for it. Just keep a watchful eye on them if you do want to mix them with Kribs.
- In the middle-to-upper regions of the tank, fish like Pencilfish, Rasboras, Danios and certain species of Tetra can work. Congo Tetras are beautiful, fast-swimming and fairly robust. Certain Barbs or even Rainbowfish can also work because they are robust and fast-swimming, but they will be on the look-out for fry.
- One of the benefits of maintaining a pair in this setting is they are much more likely to bond properly than a pair being kept on their own. Because they've got other fish to focus their aggression on, they're less likely to turn on each other.
- Once again, loads of plants, wood and rocks to break up the lines of sight, and the position of the Kribs' cave is key.
Remember: although it definitely can be done, you could follow all of the above but still have a pair going round attacking anything they see. Just be prepared to observe intently and remove fish that are causing problems.
Brilliant, thank you
just one other thing, I've got coconut shells as caves but what's the easiest way of creating a opening?
There's a really easy method I've used in the past:
Firmly tap along the equator of the coconut with a hammer (or whatever) whilst rotating the coconut. It may seem like nothing is happening but, trust me, eventually it will break in two, and it will be a nice clean break.
yesterday I went to one of the fish shops in Cardiff and they had ceramic ones with a neat round hole, so there's 2 of them in the tank now as well. 
