I’m a bit late to the party here, so dsac may well have already found answers, but thought I would share some answers here in case it’s helpful to others as well. :)
What are the specific filtering options
It’s not currently possible to filter the RSS items directly within the trigger step. An additional Filter by Zapier step would need to be added to filter the RSS items by specific categories or keywords etc. More details on how to add filters to your Zaps can be found here: Add conditions to Zaps with filters
What limitations
The RSS by Zapier app has a New Items in Multiple Feeds trigger event which would allow you to trigger on a maximum of 10 different feeds in a single Zap. I’m not aware of a specific limit to the number of RSS items that can be added within a specific timeframe for the Zap to trigger on them but our Flood Protection does kick in at 100+ items so may be something to keep in mind!
What are the Zapier fees and how are they determined
The costs of using Zapier wouldn’t be linked to the number of feeds or posts specifically. It generally comes down to the type/number of Zaps you create and the number of Tasks they use up each month. You can find out more about how Tasks are counted here: Learn about tasks in Zapier
And you can see the pricing and compare the features across each plan here: https://zapier.com/app/billing/plans
Feeds updated on time schedule
Both triggers for the RSS by Zapier app are polling triggers so they would check for new items every 1-15 mins depending on the Zapier plan you’re on (and would only trigger once a new item is found). For example, Free and Starter plans are have an Update Time of 15 minutes so the Zap would check for new RSS items every 15 mins.