I'm new to CAP, and I just want to make sure I fully understand the concept of CAP. Each CAP pokemon is made for usage in OU without mention of any other CAP. Say Tyranitar was a CAP. When the building process leads up to the counters, one should not mention Arghonaut because it's another CAP?
Yes, that is correct. We do not consider previous CAP pokemon when building a new CAP pokemon. We treat each new CAP as a singular addition to the OU metagame that is current at that time.
This would also mean that the full CAP metagame, where all CAPs are used, isn't as ballanced as the playtesting CAP metagame where only one CAP is used?
Yes, that is correct. The full CAP metagame, which is played on the server in between playtesting periods, is not necessarily balanced.
The purpose of the "full CAP metagame" is to provide an interesting diversion from OU, for players that frequent the CAP server. But the full CAP metagame is not something that we consider to be a primary part of the overall educational goal of the CAP project. We are not trying to build a new balanced metagame with multiple new pokemon. That would require considerably more work, planning, and processes than we can reasonably manage. The full CAP metagame is simply the byproduct that results from creating several individual CAP pokemon.
However, since there is a large number of server regulars that enjoy playing the full CAP metagame, you may see articles on the CAP website and threads in the forum that pertain to the CAP metagame. Also, we sometimes alter previous CAP pokemon to suit the desires of the server community. Sometimes changes are made to accommodate metagame changes, like HGSS. Other times we alter pokemon to make them more or less powerful in conjunction with other CAP pokemon. We call this the "CAP Revision Process". There are no rigid rules regarding revisions, since an overall balanced CAP metagame is not the focus of the CAP project. Revisions are normally conducted on an ad hoc basis, at the request of the CAP server community.