Is it acceptable to start less able children on easier items?
Yes. The authors emphasise the importance of doing this in their discussion of starting points on page 21 of the Administration Manual. This avoids any possibility of children being discouraged by their inability to pass at least three of the items intended for their age. It is also legitimate to give children the lower level of the GCA, if you think that the scales specified for their chronological age would be too demanding (see p 18 of the Administration and Scoring Manual). For example, a six year old could be given the Early Years Upper Level and a four year, the Early Years Lower Level.