Three items to note.
1 - Where there are more than 2 tracks, at least one of the intervals should be a 10 foot way. The GWR were quite lucky on this as they kept the old Broad Gauge interval in a lot of places when extra tracks were laid on either side.
2 - In 00 the track gauge is only 4' 1.5". Given that the stock is the same width as on 4' 8.5" gauge on the 1:1 railway, the 6 foot needs to be slightly wider. Using track centres is a more reliable way of avoiding side-swipes.
3 - Don't forget that because of non-prototypical curves there is extra side and end throw, particularly on bogie vehicles and locos. The minimum Passing Clearance between vehicles should be 18 inches at 1:1 or 6mm in 00.