i see a good deal there. cant beat 5 buckeroos.
i also see a problem with the pleco, since most plecos grow to about 10''+
and need a 75-100+ gallon tank.
im not very knowledgible on cichlids, but im pretty sure males are aggressive and territorial ( could be wrong about that )
as for substrate, i would suggest eco-complete or sand with added root tabs. this allows your roots to easily grow.
as for your plants id suggest browsing www.liveaquaria.com
to get an idea of what types you like, a price range and what lighting requirements you will need to meet.
i suggest for a 20 gal. a few different medium (mid ground) plants for your back and a few small (fore ground) plants for your front (mosses,dwarf hairgrass,dwarf baby tears all make nice,short plants for the front of your tank), and you may be able to do one or 2 large (back ground) plants against the back wall.
a piece of malaysian drift wood is always a nice touch
whats the piping connecting the two tanks?
what kind of filter you plan on buying?
dont forget to CYCLE the tank :D
def. dont forget to keep us posted