Yes, you can take mix armor types. Though the entry's language is a bit ambiguous, its listing qualifiers for entry.
"The Caestus Assault Ram has a transport capacity of ten models which must either be in power armour, artificer armour or terminator armour."
1. Is the terminator wearing terminator armor?
> Yes, therefore he may be transported.
2. Is the chaplain wearing power armor?
> Yes, therefore he may be transported.
At this point, both models are granted access. As there is no language barring a mix of armor types, they may be transported together.