(1) Mixing different types of metal track will work, but brass requires more maintenance than any other type of track.
(2) The brass or LifeLike track will almost certainly be code 100.
(3) The LifeLike Snap Track can be either nickel silver or steel, they make two different kinds. Gray roadbed is nickel silver, black roadbed is steel, just like Bachmann's line of EZ Track.
(4) No. The track will not separate from The LifeLike snap track. The only roadbed track that is removable, to my knowledge, is the Atlas TrueTrack.
Before you buy any track, answer yourself this question: Is this layout going to be permanently mounted to a board, or will you have to be undoing the track frequently? If it is going to be permanently mounted, then I would start off with the Atlas sectional track, the non-roadbed kind. If it will have to be separated frequently, I would really recommend going with Bachmann's nickel silver roadbed track with the GRAY, not black, roadbed.
Different brands of roadbed track will not interchange with each other. The connector ends are different. And even the Bachmann gray track is slightly different in height from the black track. So after you decide if it is to permanently mounted or separated frequently, pick a type of track and stick to it.
Good luck!
