You don't need yellow fever inoculation if only transferring at Nairobi...check with Tanzanian High Commission.
Tipping is obligatory...take advice from your guide.
The walking day on Kili is 25km (half of which is obviously uphill!). Strenuous for anyone....our 8 & 9 yr olds made it but it was tough for them.
Learn some local lingo - everyone will say hello ('jambo') if you say hello to them.
Expect a couple of hours drive to get anywhere...the roads are slow and bumpy but there was plenty of space to stretch out in the buses / land-cruisers and so much to see out of the window.
A good set of binoculars is essential for every person on safari.