What MySQL storage engine and table schema should I use?
If you're updating frequently, you'll want innodb, which supports row-level locking.  Your stats can go in a different table, which will be more efficient since it's not as wide as your 'feed' table.

Your data types look pretty good (unsigned, small/tiny).

I'm not a fan of your column names.  Why not 'feed_id', etc.?  Will you have no other "noun" in the system that could be abbreviated as "f"?

If f_cat is a foreign key, it should be cat_id.

There's basically no difference between a VARCHAR(50) and a VARCHAR(255), so you might think about giving yourself more room if you might need it.

Since f_lang is NOT NULL and only two characters wide, it's a small waste to make it a VARCHAR.  I'd prefer to see it as a CHAR(2), which saves the lookup.  Or normalize it and make it a foreign key over to a language table.

You've only indexes f_id and u_id.  Is that really all you'll be coming in with?  If not, think of composite indexes that match your where clause.

Hope that helps.