Theoretical characteristics and harmonic treatment are specific to each musical period. Armed with this knowledge, you can interpret when a piece was written by only studying the music. You must remember this when you are comparing other composers' works with your studies. Your last couple questions concerned Baroque counterpoint, and in this question you identify Beethoven, who bridged the Classical and Romantic periods.
Beethoven is famous for playing with expectations of convention, such has form and cadential treatment.
You will find exceptions to every rule. Understand that each rule you learn reflects a specific musical characteristic for the time period. In the Baroque, V often followed a I6/4. Remember that there are several different types of cadences, and a cadential six-four still functions as such if the following chord results in any type of cadence. If it does not, then it may considered a chord with some passing tones, or even in Beethoven's case, a random chord.
In another question you mentioned that you know Bach's chorales. I think if this were true, then you wouldn't have needed to ask this question.
 Bach regularly employs all different types of cadential progressions throughout his chorales, and this flexibility allows him to wander quite extensively through different keys. This is one of the reasons why his chorales are so rewarding - because they are so harmonically rich with activity. It is very common for Bach to drift through five or six keys in a given chorale.
In order to keep things fresh, one technique Bach typically employs is use of chromatic passing tones to set up a deceptive cadence by way of suspensions and resolutions, instead of the expected authentic cadence.
My comment related the thought that if you had an intimate understanding of Bach's chorales like you alluded to in the other thread, then you would have understood this information already, and would therefore have rendered your question unnecessary.
To answer your question directly, analysis of mm.7-8 in Beethoven's Piano Concerto No. 1 in C Major yields thus:
V, I6/3, V6/4, I, V7, I
A cadence is a period on the end of a musical sentence. In m.7, the V6/4 cannot be considered a cadential six-four as the cadence occurs between V7-I. This analysis is supported two-fold:
- It is in mm.8 and early Beethoven did things in groups of four like many composers.
- A contrasting motive begins on the "+" of beat three in m.8, thus beginning the consequent to the eight-measure antecedent constructed from the beginning of the piece.