Currently CustomerID's start at 1 while the first Order generated has OrderID 100000001.
Is there any way of altering these fields, so the first customer created has
customer number 900000001 and the first order created has OrderID 900000001?
By changing the increment_last_id in eav_entity_store table, you can control the starting number of customer number, Order Number, Invoice Number, and shipping Memo Id.