Excursions: you are not required to book your tours through either Signature or First Choice. Other distribution channels offer the exact same tours. So, if you are booking the same tour your insurance will cover it.
HOWEVER - beware of the CHEAP excursions offered on the beach and elsewhere. They cannot offer a properly licensed and insured tour with licensed guides for that small amount of money! Make sure when booking tours that they are fully licensed and insured.
I will repeat my earlier advice - if your rep gives you good service and offers a decent price - book with them. IF they don't then take your business where you choose.
I personally can speak of the professionalism of Martin Espinal at Isaira Tours. They offer pretty much the same tours as First Choice and Signature.
Isaira Tours - Dominican Republic
YEs Monster Truck is good and safe.(about 89US) So are many of the jeep safaris. I love the Catamaran trip offered by Freestyle. Stay away from the party boat Catamaran. It isn't worth it. There are many tours to choose from, find what you like.
The beer of choice is Presidente. The rum of choice is Brugal. IF you like dark the Extra Old is very nice and very inexpensive. They also make a white which I prefer. Also buy cinammon, nutmeg etc here. And of course vanilla for cooking.