It doesn't matter what capacity pack you use, your charger'll charge them up... it'll just take longer for bigger packs. You can (and probably should) use a 6V pack instead, and use NiMH instead of NiCad... you can charge them with the same charger, but of course you'll get better results & more info with one of the new computerised chargers - you can pick them up cheap on ebay from Hong Kong, plus they let you discharge, cycle etc automatically. The 5 cell NiMH pack will weigh as much as the 4 cell NiCad one, it won't have the same "memory effect" as the NiCads, and you'll get better results from servos and receivers. I personally wouldn't fly with anything less than 2200mah pack but I know of plenty of people who do. If you can solder you can save a bunch of cash on batteries by buying individual cells (with solder tabs) and soldering up your packs yourself.

Yes, you can use 2 switches in parallel and 2 battery packs as well - it's called a redundant power system. Make sure you use the same voltage packs (eg: 4.8V or 6V) and try to use similar capacity packs (eg: don't use a 700mah + 2200mah) to avoid one pack trying to charge up the other pack. You can connect them to your receiver with a Y adapter after the switches, or if you have an unused channel just plug the second switch into that. Be aware, you'll need to balance the desire for redundancy with the desire to keep the weight down as much as possible. Don't forget to re-check your CG if you add a second pack to a plane that's already set up.