My thoughts:
1) Much better with a solo pair of apisto's.
2) That pH isn't too high but Elizabethae probably won't fair well with it in the long term. I know at least where I am most Elizabethae tend to be wild caught pairs (around $185 I've seen them for, a fair bit to fork out for incorrect water parameters to kill them). From my knowledge Agassizi's will adapt better to this pH, I prefer Elizabethae though... up to you. A low pH is pretty achievable though.
3) More Diamond tetra's, aim for 8 or 10.
4) Corys are fine IMO, some people will suggest that they will hassle/be hasseled by the apisto's
I beg to differ, ph is NOT easy to lower...unless your tap water has a low KH than it's near impossible to lower GH and PH without using RO water. The KH acts as a buffer and makes peat and IAL useless.
If you want to go the biotope looking route this is what you should be aiming for:
[VIDEO]https
/www.youtube.com/watch?v=c3PXChKZ8SA[/VIDEO]
Obviously with a more black water look to your tank though.
Also keep in mind, not many, if any plants grow in true black water environments.
