Unit-type question

How to create a group of unit-types that would be treated as one unit-type?

You could create custom units or modify existing ones. Make it so that they share the same classification, something that no other units that are in the map are. Then check the classification of the unit.

Or create a trigger that saves some value to a hashatable for all units of the types you want to group. Then check the value.

What are you using it for if I may ask? Knowing that helps to find out a solution.

Or you can create them with IDs like 'u000' - 'u00A' in one group, 'u00B'- 'u01D' another one and so on... you can't edit the codes normally, but imho it should be possible by opening MPQ and editing the unit file