An extraneous unit must be built into two systems: "system of increasing recruiting time" and "system of Hun federates".

If you do not, then you will break the balance in relation to vanilla units.

Units are distributed in sets in Table. Unit_set_to_unit_junctions_table
See Attachments L1-L4. It is clear that L1, this is a group of units, which adds a +1 turn of recruitment. Accordingly, L2 L3 L4 (+2, +3, +4).
New units must be distributed according to these sets according to the system:
2 level of units +1, 3 level of units +2, 4 level of units +3.
Units, especially cool for their niche, in the right group at their discretion. For example, Celtic berserkers, this is level 1, but they in L1
Elite and sub-elite from +1 to +3 depending on the TTX and availability (the same Celtic berserkers seem to be a sub-elite).
In the L4 (+4) tab, the coolest units.

In the Aet1 tab, a group of units is all non-Guns. Here it is necessary to add units that can be potential federates from the Huns. Through this group, the cost for maintaining the feds for the Huns (through their incidents) is regulated.

If the text of this manual is difficult for you to understand, then let me know. I will then work on it in Google translator.