The "A" and "B" designations refer to frequency ranges. The "A" range is 516 - 558 MHz. Within a frequency range, there are different channels. It would have provided you more potential flexibility to have one set (body pack - base station) in the "A" range and the other in the "B" range, but you should be OK. I think it would be best if you ran one set at a channel assigned to the low end of the A frequency range while the other set is assigned a channel at the high end of the range. This way, the frequencies used are spread apart. If you try to use the same channel (and frequency) for both, then they will fight each other and audio quality will be terrible. By spacing them as far apart as you can, you eliminate the frequency contention. Read the manual that comes with the wireless lavs - they will guide you through auto and manual channel selection and assignment.
Also, I will be needing a shotgun mic and the best quality I have seen for the buck is the Sennheiser ME66 capsule with the K6 powerstick. Is this the best or is there another that is just as good but cheaper??
It depends what you'll be using it for. Sennheiser makes great equipment. The pairing you have is fine for picking up audio no more than about 8-12 feet away from the mic elements. But if you use the mic for some further distance, perhaps a "parabolic mic" is in order? My take is you want to record waterfowl audio and that will be (possibly) many dozens of feet - or more - away from the mic.
Last question, I will be needing a light for the night scenes and I don't really want to use a light source that weighs a ton and or you have to use a belt pack to power it. Are any of the cheap, hot shoe mount lights any good? And if so, which ones? LED or haloolgen?
It depends how far away the subject needing to be illuminated is from the light source. If the subject is within about 8 feet of the light source, then a low end camera mounted light might be enough. If it is further away then that, then the light source needs a longer throw - and more power to do that.
LED lighting uses a lot less power than other types - but it has a blue tint so be sure to use the camcorder's white balance capability. The cheap hot shoe mounted lights can work. Generally, they will have a short throw. And they will reduce the camcorder's video capture time if they use the camcorder's battery for power - so you need to adjust accordingly. You will likely need an additional, optional high capacity battery. If the video light is merely using the shoe (and not camcorder power), there are a few that have a built-in battery - and typically short (15 minutes?) lighting time. Some can use batteries that the camcorders use. I'm happy with the Dot Line 60 on-camera LED video light/barn door system I use - it uses the same batteries my Sony HDR-FX and HVR-V and Z series cameras use. I am not aware of their using Panasonic camcorder batteries... BUT, at this relatively low power the throw is only about 8 feet maximum - if you need further, then you need brighter - and more power - and belt-packs or mounting battery packs below or on the camera. The halogens provide a more natural light - but they get HOT. The LEDS are blue-ish and don't get hot at all.