I think the yeast from fermentation of a beer like this probably got beat up pretty good, especially if you had it in secondary for a while. You certainly don't have to add new yeast at bottling, but for a big beer that you want to put a little age on, I think it's cheap insurance.
I save a little yeast from a starter in a sterile cup or make up a small starter to get a slurry and keep it in the fridge for a week or so. I time the beers so I can bottle two or three over a two week period using a bit of the slurry at bottling time.
The other cheaper and easier option is a pack of S-33. Add maybe 1/3 of it rehydrated to your bottling bucket. I don't save dry yeast when opened.
The second part of your question about champage bottles- for convenience, I would treat it as two separate beers. Prime and bottle the longnecks, then with what's left, measure the volume to be bottled and prime accordingly. You could also prime it all, say to 3 volumes. Then measure the amount remaining and figure how much sugar you need to add more CO2 just for that.
So, to get 3 volumes of 5 gallons you add x grams sugar. You bottle and now have 2 gallons left for champagne bottles that you want at 4.5 vol. So you need enough sugar to get 1.5 more volumes CO2. Plug it into calculator and see what amount of sugar will make 2 gallons get 1.5 volumes.
The problem with the latter method is it assumes perfect distribution of the sugar which may not be the case. But you have some safety because you are using rugged bottles at the higher pressure.