Great idea Herm!
General rule of thumb for all "steel" tanks goes like this.
They're backwards compatible. If you've got an older bike and are trying to fit a newer tank it will fit, but might need a few mods or a change(s). Unfortunately putting older tanks on newer bikes will take more work than most will want to do/bother with for either PITA factor or $$$.
Specific differences are below
from 1993 - 1999
All the carbed tanks fit the same (but the cutouts for the seat are different from the newer 2000+ designs) and they only have 1 fuel line coming out which uses either 5/8" or 8mm fuel line. All steel tanks have 2 drain lines which are 1/4" or 6.35mm sized lines. All carbed models have their fuel filters/pumps outside the tank. You can use a newer tank, but you'll have to also use a newer seat to fit the cut out of the tank (it's only a little off and if you're not a perfectionist, it's livable)
In 2000 & 2001
Ducati made both carbed and FI tanks. The most obvious difference between them is that the fuel filler on top either has just a fuel cap (for carbed models) or a fuel cap which is just inside an aluminum ring (for the FI models).
All FI tanks have their fuel filters and fuel pumps inside the tanks. And you would remove both the fuel cap and the aluminum ring which the fuel cap mounts to from the tank (gently using the wooden handle of a hammer) to gain access to both the fuel filter and pump. It helps if you've got small arms to fit inside that hole.
On the bottom of these tanks there are 4 lines coming out. All 4 of which are bent at 90 degrees. 2 are large for fuel (5/16" or 8mm) and 2 are small (1/4" or 6.35mm). The cut out underneath for the airbox and battery are different than the newer design and will NOT fit newer tanks without a lot of relocating of parts or modifying.
These are the rarest tanks as they are for carbed bikes and have the newer seat cutout (which fits everything from 2000 to current in the Monster line), but are have the smaller hole at the top of the tank for only a gas cap and NOT the extra aluminum ring. But it's fairly easy to make any FI tank fit this bike with a little work. Closing off one of the fuel lines and adding the extra aluminum ring (which didn't come with the original carbed tank) and has 8 2mm allen bolts holding it into the tank.
All tanks from this point forward are FI only.
The 2001 S4
tank is special as it has the underside shape of the newer 2002+ style tanks, but it came out in 2001. (FYI- this was the first Monster to use the ST style frame and 5.9 "immobilizer" electronics, all Monsters going forward use this)
From 2002 & 2005/2006
Ducati also went with a different location for the battery and airbox underneath the tank. So the cut out underneath is larger overall (hence why the older tanks don't fit) also the fuel filter is held inside the tank with a C clamp in these tanks, rather than just floating in the previous FI tanks. The easiest way to tell the difference between a 2000/2001 FI tank and a 2002/200x FI tank is by the fuel lines coming out of the bottom. Specifically the 2002/200x tanks have 4 lines coming out of the bottom, but only 2 are bent at 90 degrees (the smaller 1/4" or 6.35mm ones) are bent at 90 degrees and 2 the larger (5/16" or 8mm) ones point straight out of the bottom towards the ground when on the bike.
In 2005/2006 (in the US at least)
all the tanks went to plastic. (But there's still a few Metals that were being depleted from stock, hence the 2 year span to actually completely change to plastic) simple check to confim, just check to see if a magnet sticks to it. These tanks have the same cutout underneath for battery and airbox. But have a different fuel filter/pump system that bolts into the bottom of the tank. They only have a gascap on the top of the tank and they use a different mechanism for the hinge.
From what I hear, the S4RS tanks are very differently shaped too. But I've not seen one to tell you what those differences are compared to others.