You will certainly need at least a Class 2 (Rigid HGV) licence, if you want to pull a trailer then a Class 1 (Articulated HGV) licence is required. As for experience then that would be learned 'on the job' so to speak. Like any vehicle they all behave differently, two cars off a production line made to the same specification will be a little different, steam is no exception to that rule. As an example the Romney, Hythe and Dymchurch Railway have 7 locos with the same cylinders, wheels and boilers (even though two are Canadian in look) and they all behave in very different ways. We now see steam as an exception but when these Waggons were common so was steam, almost every man who worked (not being sexist here but 90% of the population in heavy manual work was male) had at sometime been party to a steam powered machine. Today I believe that given a Waggon and a week I could get you up to speed as a Fireman on one of these, two weeks and you would be driving. However that would be 12 hours a day for five days and at least eight on the sixth, you can have Sunday off though.
For me, I grew up with Steam machinery, it was Dads work and Hobby, steam is normal and I struggle to understand why people see it as some mystical art. I still Drive steam locos on a couple of Railways for fun, and am in the position that I have a current HGV class 1 driving licence. I just don't have a Waggon. Basically I have been running steam plant since I was four years old (am now 52 so I have a few years under my belt) and I still get it wrong on occasion. Once you know what the bits do then the rest is down to 'feel', it is impossible to describe 'feel'. You Drive any steam machine with all your senses, you listen to the fire, the exhaust and the engine, you are constantly watching not just the road (steel or tarmac) but the machine itself, your touch is telling you stuff as well (be it your feet, hands or bum) and finally smell, the smoke, the hot oil on the engine and the steam. It is all giving you information, information that is letting you know just what state the machine is in. You will smell overheated oil before the bearing makes a noise, you can see an off coloured splash of oil that tells you that a bearing is a little tight. By the colour of your smoke you can see how good your fire is, light grey haze at the top of the stack is good. For many the first time on the Footplate/Manstand is a sensory overload. Today we are coccooned in tin boxes, seated in comfort with no direct connection to the road beneath us. I'm not complaining about that at all, I would not want to punch a Steam Artic down to the South of Spain the same way I used to my big Volvo Artic. I have no complaints about Driving a Diesel Rail loco, nice warm and dry for a change, but a Steam Vehicle is somehow more 'alive'. They talk, moan, groan and chatter all the time, they let you know when something is going wrong. They are all bad tempered and will hurt the unwary, you never get over familiar with any Steam powered machine.
I'm sorry I have wandered a bit, I'm an unabashed Steam Junkie. The point is that you never stop learning. I could get you competent firing and driving a Steam Machine in a couple of weeks, then you start learning and never stop. Ever. A Driver on one of the Railways I drive on has been Driving since he was 9 years old, he is now 84. Last year his loco was playing up and I found that a small adjustment to the fire box door made a huge difference. The owner asked me what I had done and I told him. He learned something new that day. You NEVER stop learning, ever.
Sentinel S4.