Boesemani rainbows (and angel fish) grow far too big to be kept in a 66l tank.
It would be great if you could post pics (you'll need to upload them to a hosting site like Flickr or Photobucket) of the two fish you have, as it might be in the fish's best interests to re-home them (if they're species that grow too large) or get them some company (if they're a social catfish, like a Corydoras or Otocinclus)
The usual guideline is 1 cm of fish for every 2 litres of water, but it only applies to small, slim bodied fish and you have to use the eventual, adult size of the fish in the calculations so as to give them enough room to grow; as we don't know what species you have in there already, we can't say how much space you have left!
Live plants are always better for fish. Many are very easy to grow; Java fern, anubias (those two need to be attatched to a piece of wood or rock), vallisneria, Amazon swords and Cryptocorynes are all good for beginners; if you're wanting to go and buy plants from your LFS, write those names down, and get only those, as many shops sell plants that are really house or bog plants that will die off in your tank (I know, it's naughty of them, but most do it

).