iJay. For cucumber i just cut it to size, place it in a cup of hot water for a minute and then i weigh it down using some plant weights. If you dont have weights you can a)push it into the substrate, b)put it in a cup of hot water and leave it over night (obviously the water will cool), and this should help it sink.
Blanching or steaming veggies works very well. It softens them up enough so the fish can get it. Livebearers can't really chew, remember. Another one that isn't mentioned here (!) is Zucchini. My Livebearers go crazy over that. The easiest way to feed all these is with a veggie clip. That way, you don't have to put it on the bottom and the fish can eat it a bit easier.