I honeymooned in Costa Rica last year (Guanacaste beach region, Monteverde, Arenal, Paos and San Jose). Your initial gear choices seem pretty good. If you're trying to slim down that pack, I'd take the three nikons and maybe the long Sigma.
What you use will depend on what you're shooting - Costa Rica is very varied in terrain. You'll likely shoot (i) beach and volcano landscapes, (ii) rain forest wildlife, (iii) cloud forest wildlife and landscapes (Monteverde) and (iv) local villages.
Volcano and cloudforest landscapes: Definitely use a tripod here. If you shoot color, this is a Velvia dream. If you shoot B&W, you may want to use a yellow-green filter for contrast. In the cloud forest you'll be way up and can get some amazing wide-angle mountain vistas with mist. At Arenal, if it's very active, you'll probably see some great twilight lava shots for your longest lens.
Rainforest wildlife: Handheld and monopod in the rain forest. You won't have the time or patience for the tripod, especially with the group and a nature guide (which you'll probably have). Mark is right on when he says that you'll want to shoot longer lenses in not-so-great light here. Crazy birds everywhere, but they appear and disappear fleetingly. I would shoot 400 or faster here. Winger, I think the waterfalls you're talking about are at Peace Lodge. I didn't like the nature park you mentioned (I hate enclosed animals), but the waterfalls on the trail below were great. Tons of tourists on that trail though and shooting with a tripod (for longer exposures) wasn't very comfortable.
On the road: Costa Rica is really rich with smiling locals and colorful villages. Don't underestimate the opportunity to get some great shots while traveling through the country. Also, keep your eyes on the trees on the roadside and listen carefully for a whooping sound. Seeing Howler Monkeys in the trees that line the roads is not uncommon and they are incredibly cool. They make this loud call to establish their territory. Enjoy some great shots of them with your long lenses. Just watch out. If you get to close, one of their defense tactics is to hurl poop at you. Try cleaning that off your expensive glass!
The best piece of advice I can give you is to figure out the weather cycles as soon as you get there. Costa Rica has very predictable weather rhythms that vary depending on when you go. If it's rainy season, it will rain at least 3 hours a day (sometimes much more), starting and ending about the same time every day. When I went it was always beautiful and sunny in the morning, with mists burning off by 9. Then at 11 the clouds would roll in and it would rain lightly until about 4. Then clear from 5 until the next day. Absolutely buy a good poncho to cover yourself with and some kind of water protection for your gear. Also, have decent hiking footwear. It gets muddy. You and it will be soaked without a doubt otherwise. This may be different when it's not rainy season, but they don't call it the rainforest for nothing.
Hope this helps. Have a great time!!!!