Compatibility of Time Capsule and Comcast router

It's fine - at least, it will work without issues.

You'll likely end up with two wireless networks - one from the Comcast box and one from the Time Capsule (from memory you can't turn off their wireless gateway), but since you get to choose which network to join you shouldn't have an issue.

There are three potential issues that I am aware of:

1. The AirPort base stations, specifically the 802.11ac AirPort Extreme and Time Capsule do not "play nice" with Arris/Motorola 6XXX-series of modem or gateways. Neither are bad in themselves, they just have overall connectivity and performance issues when used together.
2. By default the TC will perform as an wireless Internet router. If left in this configuration with the Comcast-provided device, you will have a "double NAT" condition. This will directly affect data throughput performance to/from the Internet as all data will have to go through, at least, two NAT routers. You can simply resolve this by reconfiguring either device as a bridge, preferably the downstream device (in this case the TC.)
3. Comcast, like some other major ISPs, have started to role out IPv6. This has caused some Internet connectivity issues. Comcast has released a note about using their IPv6 service with Apple routers ... but looks like the link to it has disappeared so I'm afraid that I can't provide it to you.

In addition to the other comments above, something else that you might want to consider.

The current version of the Time Capsule was introduced in June 2013. Other than a few firmware updates, Apple has not updated the hardware in over 3 years.

Since it normally takes a year to develop a new product, the current Time Capsule represents 4 year old technology.

So, if you are not in a hurry to add a new product, you might want to wait and see if Apple will offer a more up to date product.....before you buy the current version.

I have an Xfinity (Comcast) Modem/Router made by Arris connected to a 2GB Time Capsule (TC), and the TC actually has a little faster download speed than the Xfinity Router.

Just remember when you connect the TC to the Modem via Airport Utility that you set the Router Mode to Off (Bridge Mode) via the Network Tab.