Only Era Units are available to be used in the Coliseum. This will include units from the current Era, which you can level up, and loaned units to fill out your squad. 

Loaned units can only be upgraded whenever you receive a Loaned Unit Era Level Increase token, which is automatically consumed to level all of your Loaned Units by 1.

For this first era, they will be provided primarily via daily inbox messages to make sure those who show up later in the era can still claim them all and aren’t permanently behind.  For future eras, we are looking into ways to improve clarity and the user flow for obtaining the Unit Era Increase tokens