Good questions. First, the software allows you to add questions anywhere within an ACBC survey. So, if you created some fixed CBC holdout tasks using a CBC exercise, or a Free Format question, you could insert them within an ACBC exercise.
Next, when I've done this, I've put the fixed CBC-looking holdouts after the ACBC questions. And, I've told the respondents that we (or a panel of experts) created the holdout questions prior to interviewing them, so they may see some levels that they previously said were unacceptable. And, that this was expected and OK.
Next, holdout tasks are not always needed. It just depends on your reason to do them. Most practitioners don't use CBC-looking holdouts alongside their ACBC questionnaires.
If you are planning on using holdouts to try to compare different model formulations or for some other (typically) academic reason, then you'll need lots of them (typically 5 or more, otherwise you have too few holdout observations to discriminate between good and not-so-good models).
If you could afford it, the best approach is to interview an entire different group of respondents (drawn to match the same characteristics of your ACBC respondents) who only receive the holdout questions and each completes about a dozen CBC-looking tasks. You create a market simulator using the ACBC respondents and (after tuning the Exponent down to something like 0.2 to 0.4) you predict the shares of choice for the CBC-looking holdouts.
Also, calibration section in ACBC is not necessary, unless you want to rescale the None parameter to be something specifically tied to a researcher-specified point along the 5-point purchase intent scale.