For FBRs, should I use a primer/base? I've been looking at white gesso, but I'm not sure if that's what I should be using.
I use white gesso. You don't have to use a primer but I like the extra tooth it gives so I always recommend it. 
Do you thin your gesso at all, or do you use it straight from the tube?
Straight from the tube/jar usually, but I believe the thickness varies between brands... I think you have to follow your gut! I slightly wet my brush every now and then, though, especially if the gesso starts to go cakey.
I only now noticed your earlier question about gradients. Make-up sponges work quite nicely but you can use brushes, too! I kind of just dab with my brush while the paint is wet... For smaller areas, or to perfect the gradient effect, I use the dry-brushing method. Take a soft, round brush (not your best one! it will take some damage in the process...) and just the tiniest amount of paint and vigorously brush it on until the edges start to fade. Repeat as many times as needed to build up a nice, soft gradient.
This method works best on top of dry paint, while the dabbing method is a wet-on-wet technique. It's also possible that the dry-brushing method does not work with all brands of acrylic paint - Folk Art paints are kind of "chalky" while some artist grade paints I've tried are more rubbery. You'll have to experiment and get to know your own paints and tools. Of course, hand-brushed gradients will never look as subtle and perfect as those done with an airbrush. I guess you could use chalk pastels too, but they will probably require a spray sealant.
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