No reason not to use both! I'd guess most people do. Some devices are a lot easier to find in one protocol vs. another (or have a feature set, appearance, price, or something else that you prefer). One of the great things about a hub like Hubitat is that it can tie together all these devices of different protocols (Zigbee, Z-Wave, LAN/Wi-Fi, cloud, etc.), as long as they are supported. You've discovered one area where one protcol has many more options than the other: there are a multitude of Z-Wave in-wall switches/dimmers out there right now and very few Zigbee ones, and many of the newer Z-Wave ones (Inovelli, Zooz, and even the older HomeSeer ones) tend to have a lot more features. If you do have your heart set on Zigbee, Inovelli has mentioned that they're working on Zigbee ones with a similar featureset to their new Red/Black (i.e., gen 2) switches, and I think late this year might be their target (but I'd plan on waiting a while longer just in case 
). Smart bulbs tend to be an area where there are lots of Zigbee options, including Hue, though it's worth noting that Inovelli also just released two (color and color temperature) Z-Wave options that are reasonably priced and work well. I still prefer Zigbee for lights. I also prefer Zigbee for motion sensors because they tend to react faster.
So, to answer your specific questions:
A) As above, no need to choose! However, both Z-Wave and Zigbee are "mesh" networks and will work best if you have at least a repeater or two for each protocol (assuming you use it), more if you have a larger area to cover. Most powered devices like switches and plugs are repeaters. See How to Build a Solid Zigbee Mesh - Hubitat Documentation and How to Build a Solid Z-Wave Mesh - Hubitat Documentation for more.
B) Nope! (Aside from the above.)
C) This depends on the exact switch you use, so you'll have to consult their documentation. Off the top of my head, GE and HomeSeer require special "add-on" switches in three-ways for other locations, Zooz requires "dumb" switches but some may require rewiring (depending on the Zooz switch), and Inovelli's new (Red/Black) switches can work in either configuration or even with another smart switch (though some have special requirements like a neutral wire--which most switches do and is also something worth checking if you haven't already).
I'll only tell you otherwise if you want to pair them directly to Hubitat instead of using a Hue Bridge. See the end of the Zigbee mesh docs I linked to above for more on this. (Most of my lights are Hue, and they are my favorite. But I use Hubitat's LAN-based Hue Bridge integration. Actually, I wrote my own, but used Hubitat's before that. Either works well.)
If you feel like spending lots of money, there's no reason you can't use both smart switches and bulbs together. I have some Inovelli switches that with local control disabled (this functionality is key unless you want to bypass this in the switch wiring; Inovelli and Zooz are two that I know let you do this) whose only purpose is to control Hue bulbs, which I do via Hubitat. This gives me smart bulb functionality (color/color temperature changing, etc.) with a traditional switch feel for guests (or me if I feel like manually adjusting something my automations don't handle). In other places, I use Pico remotes to simulate switches. You probably don't want to start here--if a switch/dimmer is enough to handle your needs, that's even easier (and cheaper than replacing both switch and bulbs) and already usable for other people, and if you use Hue, a variety of accessories they make or "bless" (Hue Dimmer, Hue Tap, Lutron Aurora) can act like switches and are easy to set up through Hue, though most of those don't easily replace a standard wall switch if you're worried about people still using those.
I think it's something we grow into as we are falling endlessly down the Rabbit Hole. It comes right after the initial screaming.. 
