It depends on the gender of the baby in question. 'infans' can be taken as a masculine or feminine noun, depending on the sex of the baby. It is declined the same way in both cases, so the use of 'infans' is correct regardless. However, the adjective 'economicus' must agree with the gender of the noun it modifies, in this case, 'infans'. Therefore, if the baby is male we have 'infans economicus' but in the case of a female baby we have 'infans economica'.

Presumably you are using it in a general case, not specific to a certain child, so the male form 'infans economicus' is fine.

The plural form of Homo habilis is Homo habilis, the plural form of Homo erectus is Homo erectus, and the plural form of Homo sapiens is Homo sapiens.

There is some dispute about the "seven" names. However, we do have "homo sapiens", "homo neandertalensis", "homo heidlebergensis","archaic homo sapiens", "homo ergaster","homo erectus", and "homo habilis".

homo sapiens idaltu

Homo sapiens probably evolved from Homo erectus, while Homo neanderthalensis and Homo floresiensis evolved separately from different branches of the Homo lineage. Homo neanderthalensis and Homo floresiensis did not directly evolve into Homo sapiens.