No, you shouldn't boil it (I used to work in a Sports Shop and dealt with Mouthguards on a daily basis).
Boil the Kettle certainly and pout it into something that you can also drop the Mouthguard into, but allow the water to cool off slightly (Even if it means adding a little cold water) so you don't scald your gums. Leave it in the water for about 10 - 15 seconds and then pop it into your mouth. Use a finger and thumb either side of your teeth and mould it to your gums - this might prove difficult because it's a double guard. Have another container full of cold water beside the hot/warm one, and when you're happy with the fit, take the Mouthguard out and drop it into the cold water for 10 - 15 seconds, and it should stay in shape. If not, re-boil the Kettle and repeat.
The main advantage of wearing a Scrum Cap is you will not end up with Cauliflower Ears (A major problem with players in the Scrum). If you're a Winger then you wouldn't necessarily need one. After receiving a nasty head injury, Arsenal Goalkeeper Petr Cech wears one for protection, so you may want to think about one. You want it to be a close fit without it feeling like your head is being squeezed, and you want the chin strap to be fairly tight without choking you. That way it won't get pulled off your head if someone grabs it.
Should I also buy baselines top and shorts and maybe padding?
Not too sure I know what baseline tops are. Do you mean baselayer tops. I swear by mine and I do no sport other than Cycling to Work. They are great for keeping the cold out (Nike Pro Combat Hyperwarm and Under Armour Copldgear, for example) and they help draw the sweat away from your skin and it holds it in the outer layer. As it warms up then you can think about switching to the thinner layers. Nylon and Polyester are quick drying too, and far better than wearing a t-shirt underneath.
Padding? Optional. You might have to put up with some hard hits, but, they MIGHT help to avoid injuries. Compression baselayers will also help - hamstrings really hate cold weather!