I wouldn't put bulbs that are over the rated wattage. You're risking a fire. I don't have an answer to your question about the short bulbs but I'm researching it myself as I have one spare fluoro fixture that I'd like to put to use.
fire doesn't have to come from explosion. the plastic shielding around the copper could slowly degrade and then spark. in fact there are probably a hundred reasons that it's a bad idea.