Well, if there is something leaking current onto "ground" and that thing is connected to your metal box, and the metal box is not connected to the ground in your breaker panel, no, that's not great. I would take a multimeter, turn off the breaker to the house, and check the resistance between neutral and the box. If it is large, then they are not not bonded together at the panel, but yet something somewhere is connecting them in your house. Try using a 3-prong tester on some outlets in your house and see what results you get.
Wire nuts come in a lot of different sizes and styles, just look at this PDF. This brochure is from Ideal and go to page 20 for the wire size chart. I used information from Ideal not because I think they are better than others but I'm more familiar with them.
@BMitch is right about mixing AL & CU, but there are wire nuts for those applications as well. Most of these say that you CANNOT use with aluminum to aluminum. Weird.
While I don't think its a problem on this site, you can use most wire nuts with 600 volt maximum. Normally the colors dictate what sizes you can use with what wires and a lot of times these will overlap one another. Yellow, tan, red, and the large grey will allow you to use 2 - #12's with each. Yellow allows 1 to 3 #12's while the large grey allows 2 to 6 # 12's. The colors and the represented wire sizes accepted is pretty much accepted by most manufacturers.
Direct burial wire nuts are available. They are 'gel' filled and have a thicker shell. I don't know if I would direct bury one except for landscape lighting, but they are ideal for being used in damp, moist areas like pole lights.
Anything larger than the listed size should be used with split bolts, which gets treated completely different.
Best Answer
It really depends on the material of your house and the height of the overhang.
In one extreme, if you have a brick house with no overhang you can be almost right up against it.
Vinyl siding and/or 9' overhang is the other extreme. You should probably not be within 10' of vinyl siding.
Somewhere in the middle is wood siding which probably will not burn but you might get paint bubbling if you are within 3'
As a general rule you don't want to be under a 1 story overhang which will hold heat and any sparks in.