The plans or manual for your planes should tell you which servo setup and how much torque you need. Many warbirds use a dual elevator servo because they have two elevator halves and with the bigger planes it's harder to get them hooked up to a single split pushrod with having a lot of slop in the system. Dual pushrod setups using a single servo are possible, as are dual pull-pulls. But for most of us, bolting on an extra servo is a lot easier. The setup is just a simple permanent mix. If your elevator is ch 2, you might put the second elevator servo on ch 7 and do a 100% mix that stays on all the time. That way the second servo does everything the first one does all the time.

On the mixing brands question, you can't mix brands of receivers in 2.4ghz radios because every manufacturer uses a different frequency hopping scheme and signal coding for control. I keep waiting to see a universal receiver that can be programmed for any of them but so far it hasn't happened. But servos are universal. Everything currently being made has the same plugs and same control coding, so you can buy any servo brand you want for any radio brand. As for choosing which one from the smorgasbord of choices, all that really matters is getting the right torque that your manual calls for in the right size to fit in your mounts. The tougher metal gear sets are good for planes that will be flown hard with lots of changing loads on the servos. None of the plane you listed will be that rough so don't spend your money on the fancy stuff. Digital holds its position better under load and gives you a more secure feel on the plane, but again with the birds you're planning to fly you won't likely notice the difference. So get the right torque servos (more power won't hurt a thing if you find a good buy) in analog with nylon or the first upgraded gears in one of the well-established brands and you'll do fine.