I agree that generally there are too few units (particularly ground units) in standard Civ2 at any one time.
I have actually had a crack at a Modpack/Scenario which seeks to rectify this for Civ2 ToT in order to add in the more gradual changes that we saw as a result of the continuous improvements over the last millennium and so as to allow a finer discrimination between the various intermediate units.
In addition to the standard :
Archers,
Musketeers,
Riflemen
Marines,
Mech Inf. etc.
I have included the following ground units.
Longbowmen (Much to and fro on the forums as to whether this is a valid inclusion or not)
Infantrymen (Musketeers drilled in Infantry procedures & equipped with Bayonet)
Early Marines (Circa 1700's)
People's Militia
Musket-Riflemen
Conscripts
Bicycle Troops
Machine Gun Group
Modern Infantry
Commandos
As well as
Knights (Early)
Knights (Later)
Lancers (Dragoon Contemporaries)
Scouts (Cavalry Contemporaries with some special Abilities)
Siege Tower
Bombard
Modern Artillery
Naval Artillery
and so on.
In the modpack at each stage in history there are generally at least 2 ground units in each 'niche' that can be built.
If interested for a look-see you can find it at :
http://members.tripod.lycos.com/kestrel18/Millennium/
(The rules.txt is in the file Millennium_Misc.zip)