Are all the 8 consciousnesses subtypes of the fifth aggregate (vijñāna/viññāṇa)? The name suggests so, but it looks strange to me: the first 5 of the 8 consciousnesses seem to be more related to the third aggregate (samjñā/saññā). I would be glad for a clarification.
All 8 types of consciousness described in Yogacara texts fit into the aggregate of consciousness, not perception. The confusion comes down to difficulty in the English translations for these terms.
The word translated as perception doesn't actually mean perception in the ordinary English sense of the word. In ordinary English perception means to be aware of something through the senses, but that is not the meaning of the word saññā. refers specifically to the way that something is interpreted by the mind, especially in terms of pattern recognition and memory. For example, when you look at a chair, you are aware of actually seeing something (which is eye-consciousness) and at the same time you are giving an interpretation to what it is, which in this case is a chair. This second layer of gathering things together and interpreting them is what saññā is in Buddhism.
Yes each of the 1st three aggregates seem to have some resemblance to the 8 consciousnesses don't they?
The form aggregate is the "forming" of a reality through the aid of the first 6 consciousnesses (this reality, according to modern physics, does not exist!), the sensation aggregate is like an "emotional judgement" of sensory input, whereas the third aggregate is as Bakomon suggests. This third aggregate is more accurately termed "discrimination" and refers to the mind's categorizing capacity.
The eight consciousness are naked consciousness, which are still false. On the other hand, the first 3 aggregates which can be confused for the consciousnesses are dense habits and are transformed earlier in the path. The work of the consciousness aggregate is very subtle and is reserved for bodhisattvas who are walking up the bhumis to Buddhahood.
For a more modern account on all these subjects I would reccommend the ~200 page chapter on the aggregates from Bill Bodri's manual "How to Measure and Deepen Your Spiritual Realization." If you cannot afford the book I highly recommend Surangama Sutra because knowing this topic of the aggregates will give oneself a complete understanding of afflictions that are transformed on the road to Complete Enlightenment!