1. Does Aristotle define a "first principle" in a genus?
In Metaphysics 998a20-999a23, which St. Thomas commentates in Metaphysica lib. 3 l. 8, Aristotle discusses
the problem whether genera (γένη) must be regarded as the elements (στοιχεῖα) and principles (ἀρχὰς) of things
Commenting on Metaphysics 1018b9,
Things are said to be prior and subsequent insofar as there is some primary thing (πρώτου) or principle (ἀρχῆς) in each class (γένει),
St. Thomas says (Metaphysica lib. 5 l. 13 n. 1)
the principle (principium) in each class (genere) of things is what is first (primum) in that class (genere)
2. Is there an inconsistency in the Summa Theologica I q. 3 a. 5 and I q. 4 a. 1?
The solution is that God is not simply the first efficient cause in a class of many other efficient causes, but the cause of efficient causation itself.
He continues (I q. 4 a. 1 co., Freddoso's transl.):
the first efficient principle has to be absolutely perfect.
et hoc oportet esse perfectissimum.
God "is not contained [i.e., limited*] in any genus as its principle (sicut principium)" (I q. 4 a. 1 co.); He is in the genus of efficient causes as its cause.
*Limitation is an imperfection.
Also, St. Thomas gives a few concise arguments for why God it is impossible for God to be the genus of anything
(impossibile est Deum esse genus alicuius
) in Compendium Theologiæ cap. 13
God cannot be a genus. What a thing is, but not that it is, comes from its genus; the thing is established in its proper existence by specific differences. But that which God is, is very existence itself. Therefore He cannot be a genus.
neque possibile est Deum esse genus. Ex genere enim habetur quid est res, non autem rem esse: nam per differentias specificas constituitur res in proprio esse; sed hoc quod Deus est, est ipsum esse. Impossibile est ergo quod sit genus.
Moreover, every genus is divided by some differences. But no differences can be apprehended in very existence itself. For differences do not share in genus except indirectly, so far as the species that are constituted by differences share in a genus. But there cannot be any difference that does not share in existence, since non-being is not the specific difference of anything. Accordingly God cannot be a genus predicated of a number of species.
Item. Omne genus differentiis aliquibus dividitur. Ipsius autem esse non est accipere aliquas differentias: differentiæ enim non participant genus nisi per accidens, inquantum species constitutæ per differentias genus participant. Non potest autem esse aliqua differentia quæ non participet esse, quia non ens nullius est differentia. Impossibile est igitur quod Deus sit genus de multis speciebus prædicatum.