Do I have to find a second caterer, who will prepare X amounts of meals specifically for guests with Celiacs or do I just trust that the original caterer won't cross contaminate anything.
I was also thinking about doing a second, small, gluten-free cake.
Re: Guests with Celiac's
In most circumstances, I would trust your caterer to accommodate your gluten-free guests with the same quality s/he provides to your other guests. But you need to ask about their experiences preparing and serving gluten-free meals. If they don't have a lot, then I'd be especially interested in talking with past clients about how it went. Cross contamination is such a tricky thing. Some people are far more sensitive it it than others. You need to be so careful that your caterer can address the most serious standard among your guests.
I also wouldn't automatically assume you can bring in a second caterer. Check your current contract and see what language it has about additional food providers and accommodating allergies. If you aren't comfortable with your caterer's ability to provide for your guests with Celiac's you may need to negotiate before you can hire an additional caterer.
Similar thing with the cake. If your baker can't provide something gluten free, just be sure you are contractually free to bring a second option in. I would try very hard to make it the same quality as your regular cake. But talk to your baker first.
We're also labeling everything.
As for the cake, I would ask your gluten free friends if they have a particular baker they'd recommend. I can tell you from experience that if someone isn't used to baking gluten free, it shows in the final product. My mom and I ended up doing tastings and finally she decided she wanted to make her own cake since none of the bakers we tried measured up to her cakes.