If the definition of a heavy tank is, as stated earlier
- Heavy tank have very good combat ability but poor mobility.
then the Matilda II is absolutely a heavy.
Valentine could be a variant on Medium. It was around at the same time as the Crusader.
The Crusader could be looked at as a fast, lightly armoured, medium, and the valentine as a slow, heavily armoured medium.
But the Matilda was off the scale on armour for 39 and 40, and could barely manage walking speed.
It was also impervious to all tank guns until 42, and all AT guns except the 88.
Mid war the British had the Tetrarch and Harry Hopkins light tanks.
Not many of them were made, but they were options.
Throughout the war the Japanese made light tanks, and not much else.
The americans had designs for heavy tanks (M6, M14) , but due to internal army politics and decisions to build huge numbers of mediums rather than smaller numbers of heavies they didn't reach the troops.
But they should be available as options.
The french had the Char B1 in 1940, and plans for both light and heavy tanks for later years.
So I think the tech tree should have options for light, medium, and heavies at all points.
Which one or ones to build should be left to the players.
The production system will then make the choice difficult - as in real life.
America chose lots of mediums, and didn't build the heavies.
Japan chose as many lights as they could manage on restricted resources.
Germany chose smaller numbers of both mediums and heavies.
UK made the same choice, and probably shouldn't have done.
Russia chose lots of mediums, and some heavies as well, and paid for that by not building other stuff.