Thanks for your reply Dan. The PH is running around the 7.2 mark here, and I know not to mix Africans with Central/South Americans .
I also realise it depends a lot on the personality of the fish .
At the momment, the Oscar and the JD get on really well and I'm hoping not to upset that balance ( fingers crossed ) .
Let me tell you my thoughts .......
I could possibly add another Oscar, but then I may get opposite sexes hence problems breeding/territory with others in there.
I dont want another JD for the same reason.
I toyed with the idea of putting the Convict pair in the new tank BUT they are an estalished pair and the two of them work like velocaraptors ( off jurassic park ) with anything else I have ever tried to put in their tank and my gut reaction is that despite having a much bigger territory in the new tank, they would still do that and make life a misery for the others. The Oscar is so gentle and the Cons are such little gits lol.
I may grow one of their fry out and add that later....thats a possibilty for the future.
I,m definatly going to add a catfish/plec. Any thoughts on which ?
I'm pondering along the lines of a Zebra or Raphael or try to find a reasonable sized Bristlenose.I need to research more 'L' numbers.

Which are the more gentler cichlids ? ( is there such a thing lolll )
What about Blue Acaras ? Severums ? Texas ? ( is there enough footprint ?)Geophagus ? herotilapia multis ?

You mention non cichlids ....PERKS....such as what ? That sounds good. ( bear in mind however I have been feeding the guppy fry and convict fry to the Oscar and JD so the pair of them are well used to the 'hunt/eat' pattern) so what ever they are, its going to have to be a reasonable size . Did I see Rainbows mentioned ? They are something I have no experience with but sound good.) School of tetras ? loll just a thought. The Black Skirts I had seemed to bump everything else off ...

I do realise I am set for fun and games with this tank and that I'm likely to loose a few along the way

so any thoughts are really appreciated.
Sorry this is such a long post and you may loose the will to live before you have read through it xxxxx Wizzy