As for limit / total limit, here is a thread on that topic:
http://forum.mikrotik.com/t/pcq-limit-total-limit-settings/12002/1
My $0.02’s worth on the topic - longer queues (higher limit/total limt) cause queues to be “bumpier” - your thorughput will fluctuate a lot if the queue is too long because TCP will take longer to realize that packets are being dropped. TCP will then slow down too much, and then stop losing packets, so it will try to go very fast again, which will cause more drops “too late”
As for your comment about not doing the burst thing on a single queue for the entire LAN - that is a correct understanding. The entire LAN would go fast and slow, sharing a single limit.
I would say that using PCQ in your scenario, if you want to allow bursting, would be to set the values as you have - what you want each individual user to experience, and then put the full bandwidth on the master queue so that if the number of users doing things starts to really exceed the master bandwidth, then every sub-queue is going to start sharing the reduced amount. Therefore you could set the individual “max limits” a little higher for scenarios where there are only two users at the time, they may as well get half the bandwidth, right? If the main queue knows there is only 100M available, then if you say “each user gets 20M” - this is going to get overridden by the master 100M whenever there are more than 5 sub-queues anyway.