What is the maximum limit of Pages/Batch in CE?

CE supports 5000 pages per batch. A batch may contain 5000 sigle page tiffs or 5 multi page tiffs containing 1000 pages each.

What is cross committal in a multi-IDMIS FileNet environment

When defining a batch, you select a batch entry service that controls
scanning, indexing, and committal. You can select a batch service on
your own local system or on a remote system.
Normally, a batch entry service resides on the system that receives the
documents. Alternatively, your support representative can configure a
batch service to commit the documents to another system. This is
cross-system committal.

For example, you can scan and index on system A, but configure
system A to commit the documents to system B. The names of the
document class, media family, and common indexes must be the same
on both systems. Otherwise, the retention parameters are taken from
system B, while media family name and document class security are
taken from system A.

Which one is better? To use 'DocProcessing' or 'Scan' component's settings for detecting barcode?

Its better to configure 'DocProcessing' Component settings to detect Patchcode/Bar code.

Capture with CE

Before assembly Merge component should be executed to create multipage tif as CE does not support Document conating multiple pages while IS does.