Hi,
I did this trip in May just gone and it was fantastic, you will have a great time whether there are 6 travellers (like ours) or 18. You will see some amazing animals in their natural environment, you will get up close and personal with them (not too close though, lol), your Somak drivers and other drivers in the Parks have their own language and will take you to where you need to be to see the animals, is there a negative, yes, there are long drives between Parks, but you do get to see the real Kenya, oh and I think its in one of the reviews, don't forget to wave at the Children as you pass, as for Mombasa, well I went there back in 2011 (with Just You), its alright, there are things to do and places to go, that's if you don't just want to sit on the beach.
Martin