The minimum tank size for black mollies is 30 gallons. So to answer your question, a ten gallon would help... For a little.
Have you cycled your 5 gallon? As in your water parameters read Ammonia - 0ppm
Nitrites - 0ppm
Nitrates - 5-40ppm
If you don't know your water parameters and don't have a water testing kit I recommend getting API liquid water testing kit.
To answer your question about feeding them, you don't HAVE to feed any live food. All you REALLY need is flakes. But I like to give my fish a variety of things, I feed flakes, pellets, sometimes an algae wafer. And veggies and fruit. Oh and blood worms as treats!
You don't HAVE to give them all these things every time you feed. Normally my fish get a pinch of flakes and a pinch of pellets everyday. Then two times a week I give them fresh veggies/fruits. Always make sure the fruit is really well washed before putting it in the tank. If your feeding cucumber, you just cut a little slice off, like into rounds, as if you were going to eat it. And you can buy clips with a suction cup on it to attach it to the side of the tank or just try and cram it somewhere so it won't float back to the top. And if no ones bites, then after a day or so remove the veggie so it doesn't rot and cause water quality issues. That are more then likely to happen because your tank is over stocked.
I can't stress how important it is that you get your test kit and keep an eye on your water parameters to ensure your fish would kick it due to poor water quality during the cycling process and due to being over stocked.