Hi Sarah, to address your two particular examples (second one first).....
The Accucraft Isle of Man (IoM) Viking Diesel is an all-brass model loco which has adjustable wheels to allow it to run on either 32mm or 45mm gauge track - basically, you loosen a couple of Allen Screws that hold the wheels to the axles, and slide the wheels in or out, checking them carefully for the correct gauge before re-tightening the screws. Note that this particular model is a rather expensive limited-production piece, very nice but also quite specialised as your surmised. As discussed before in one of your other threads, G scale uses 45mm gauge track (which it shares with Gauge 1) but 32mm (Gauge 0) track is more commonly used by the live steam and battery diesel people, so certain specialised models (though NOT those made by LGB) do have adjustable wheel spacing to allow use on both gauges.
As to the two Stainz power bogies, the use of the numbers here is very confusing to the uninitiated - the 45mm refers to the track gauge, of course, but the "16mm" refers to "16mm to the foot", which is actually a model scale, not a gauge of track. 16mm to the foot works out at approx. 1/19 when expressed in the more common model scale ratio; the use of "mm to the foot" as a way of expressing scale is very common in other model railway gauges too - for example 0 gauge trains are often called "7mm" as an abbreviation of "7mm to the foot" scale (about 1/43 scale) and 00 gauge is commonly known as "4mm", or "4mm to the foot" (about 1/76).
It's all part of the HORRIBLY complex and confusing mish-mash of terms that the railway modelling hobby (and other branches of modelling too) uses to define model scales and/or track gauges.....
What the seller of the Stainz power chassis is trying to say, I believe, is that the wheels are suitable for 45mm gauge track (as is all LGB), while the scale of the models would pass for use with 16mm to the foot equipment.
I hope that hasn't made it all even more confusing..... you have to remember that a lot of sellers on eBay and elsewhere don't always know themselves exactly what it is that they are selling, and thus descriptions can get very mangled...
Jon.