Well as an update - all went pretty smoothly at Miami. In fact I had the best experience ever at a US airport in clearing immigration - the automatic machines are fast. Given we were travelling with friends who were in World Traveller so we waited to go through with them, we had most of the 747 in front plus several other flights and we must have been through immigration in under 20 minutes.
The major issue in Miami was a reversal of everything I have experienced at US airports and that was security. In a word - shambles! Queued for ages, no clear organization and a mess once we got to the machines. We made the connecting flight with about 10 mins to spare.
My experience on the return flight at San Juan was also a security nightmare and don't get me started on the trashing of a suitcase on the return trip by the TSA in Miami! So clearly had nothing to worry about but the advice was excellent and very helpful so Thanks!
