I typically aim to have 1 gallon of head space for my meads. I use 1/6bbl sanke keg for fermenters, so I make 4 gallon batches in them. I mix the entire batch at the one time, getting it all in there. I also add the nutrient to the must that it should need. I've done both adding all the nutrient at the start, and staggering it, with the same results. Be sure to aerate/degass the must periodically until the 1/3 break, then leave it alone.
I've used Lalvin yeast almost exclusively for my meads (used Wyeast Eau de Vie for one batch). I match the mead/must formulation to what the yeast will handle. Never been disappointed this way.
Before you make your first batch, look over the forums on the Got Mead? site. TONS of information to be found there. Far more people there making mead than over here.