First, the difficulty of block mining is not related to the nature of transactions, or the number of transactions in that block, because the mining only applied to the block header which contains a single merkle root hash for all the transactions within. In any event, in a private network you won't be used proof-of-work as the basis for mining decentralization, but rather MultiChain's mining diversity idea (lenient round-robin).
In any event, there is no problem if mining takes a bit less or more than the target block time - that time is just the target that the nodes aim to achieve, on average.
