They get around 8 inches long, and generally need around 50, but if you are going to get a 75 early next year, and they are small now, go for it. They will grow at a decently quick rate, but they definitley won't outgrow the 30 before you get a new tank, so I wouldn't worry about it. They will be fine with live plants, but make sure you cover the roots completely in the substrate or they will most likely eat them.