It is not necessary that everything and being must come directly from the Purusha.
Once the Varnas are created from the Purusha, they themselves can now inter mix to create all types of inter castes. Purusha Sukta also doesn't mention the Varna Sankaras to come directly from some portion of him.
There are only four Varnas and no fifth one. And, all those four were created by Purusha. The Varna Sankaras are the outcome of the intermixture of these created Varnas.
Manu Smriti 10.4. Brahmana, the Kshatriya, and the Vaisya castes
(varna) are the twice-born ones, but the fourth, the Sudra, has one
birth only; there is no fifth (caste)
Similar arguments hold for the Mlecchas. They were originally created from a celestial cow (called Nandini)'s tail.
Mleccha (म्लेच्छ).—A tribe of people of ancient India. This tribe was
born from the tail of the celestial cow Nandinī, kept by Vasiṣṭha for
sacrificial purposes when there was a fight between Viśvāmitra and
Vasiṣṭha. Mahābhārata gives the following information regarding them.
The mlecchas who sprang up from the tail of the celestial cow Nandinī
sent the army of Viśvāmitra flying in terror. (Śloka 38, Chapter 174,
Ādi Parva). (See full article at Story of Mleccha from the Puranic
encyclopaedia by Vettam Mani)
There are different accounts of how the Mlecchas came into existence. Like:
1a) Mleccha (म्लेच्छ).—Born out of the left side of Vena's body when
the latter's body was churned;1 begin with Anu, son of Yayāti; ruled
by Dakṣa; ruled by Turvasu; ruled by the hundred sons of Pracetas of
Druhyu family in the east; the rule of the kings;2 not to be seen in
Kuśadvīpa; Śakas (Parādas, Viṣṇu-purāṇa), Pallavas (Paplava,
Viṣṇu-purāṇa) and Kāmbojas, all Mlecchas; (Ābhiras, Viṣṇu-purāṇa)
Guruṇdas and Vṛṣalas, also belonged to this group;3 kingdoms of, found
in the lotus coming out of the naval of the Lord;4 their kingdoms side
by side with those of the Āryas;5 people who live in their countries
are ineligible for Pārvana śrāddha;6 attain salvation at Benares;7 do
not molest the women of their enemies;8 to be conquered by Kalki;9 put
down by Pramati god.10
1b) These were the elder Madhucchandasas, who were cursed by
Viśvāmitra to become Mlecchas;1 defeated by Bharata; these were the
Mlecchas of the north;2 worship progenitors;3 eleven kings ruled for
300 years.4 Then Kolikilas followed;5 their dharma described.6
So, its not required for the Purusha to directly create them. They can come into existence much later in the phase of creation, with the basic elements needed for creation being coming directly from the Purusha.