Can an IC join a unit of Carnifexes?

[Ferro]

Okay, so IC's can't join a single model MC unit, but if the unit comprises of 2 or more, it can.

This is incorrect. And IC can join a single carnifex, or a single Tac Marine, or a single anything as long as it had the option to deploy as a larger unit.
The restriction depends on whether the unit must always be deployed as a single model (Tyrannofex, Harpy, Trygon, any unique model, etc). These models cannot be joined by an IC.
If the unit has the ability to deploy as a squad of more than one (Carnifex squad size of 1-3), then that unit may be joined by an IC, even if there's only a single model on the table.

What happens if the IC joining is a MC too? Since the MC is doing the joining and not being joined?
Basically, can a Tyrant join a Unit of two carnifexes?

I can't think of any units that are both IC and MC. If there are any, they are rare.
A Tyrant does not have the IC rule, so a Tyrant can't join anything. (Tyrant Guards have a rule which allow a Tyrant to join them, but this does not make the Tyrant an IC).

[LoverzCry]

*posted in wrong thread*