The interestingly named Maryborough Station in Victoria, Australia, has dual gauge track - 5' 3" and 4' 8½".
..bit far to run a tunnel to there, though..
Yes, but they aren't really fishplates, so Rail Joiners is a better description....
You can also get plastic ones, for where you might want an isolated section.
Cosmetic plastic fishplates are available for the right 'look'.
In 00 4mm is one foot - a tenth of an inch is 2.5 mm, or 5/8 of a foot, which is 5 times 1.5 inches, or 7.5 inches - a bit over-sized.
Somewhere around 5 or 5.5 inches seems about right, in real life.
Streamline is a trade-name for Peco's flexitrack system and its associated points, slips and crossovers. I think the term is used because you can sweep bends more realistically than with fixed-curve Setrack.
The rail on code 100 is 100/1000th of an inch high and Code 75 is 75/1000ths high. Fine scale is a generic term for things that are more scale-accurate - thus, 75 track is more "fine scale" than 100 is.
I once spent ages trying to get an old loco running on 100 to stop ticking - only to eventually find that the noise was not in the mechanism, but came from the flanges hitting the chairs..