I run trips over there and the season is great from July till mid October, earlier in the season the calves are smaller which can mean some really special images as the size difference is huge. Yes later in the season the calves are bigger and then can come close but it just depends. I know people who have had their best calf encounters in July, I am actually shooting for a big 3D project in July and then on my own trips in August / Sept as long as you are in the season you will have a great time.... no I should say a mind blowing time!
It is correct that you can only have 4 people and a guide in the water at one time but I have found that 8 is a good max number as it means that you get to get surface shots, relax, grab a coffee. It is reasonably physical getting so having a breather isn't a bad idea. I also find that in the group there are always people who are happy to sit out a couple of sessions so the keen people can get shots etc.
I have a couple spots left on my trips if you are keen, one on the first week and two on the second week, the details are here.
A couple of advantages of going with me are that I charter the vessel for the whole day not just 5-6 hours which the other operators do which means that we can leave early and return late if needs be to get the shots / time in the water everyone wants. I have never found people disappointed with the amount of time in the water they have had.... plus on our private charter it means we can go to a few out of the way locations.
No matter who you go with you will have a great time, do note though that Blue Lagoon gets booked very quickly. In fact I think they are pretty much full for 2013.