Help: Novel in 3rd person, I'm adding 1st person thoughts for MC

Nope, it's perfectly fine. It's called third person limited when you tell the events in third person but focus in on one character and can hear his/her thoughts. There's also third person objective (no thoughts, just action) and third person omniscient (knows all thoughts/actions). Third person limited is actually quite common and useful.

This what Renni Browne & Dave King (Self-editing for Fiction Writers) said:

...Another technique for setting off interior monologue is sharply is to write it in the first person (often in italics) when you narrative is in the third, a technique that is most effective when the passage of interior monologue is a self-conscious, internal thought--interior dialogue, in effect.

He broke into a spint. The car was moving more quickly than it seemed to be, but he managed to catch up to and grab the back bumper.

Great, what do I do now?
He stopped, panting, and simply watched the car sail off the retaining wall at the driveway's bottom.

Effective as this this technique can be in letting readers into your character's heads, be careful not to use it too often. Interior dialogue can easily become a gimmick, and if over used it can make your characters seem as if they have multiple-personality disorder.

Also, unless you are deliberately writing with narrative distance, there is no reason to cast your interior monologue in the first person. After all, if your interior monologue is in first person and your narrative is in third, it naturally creates a sense that the narrator and the thinker are not the same . . . .

And whether or not you are writing with narrative distances, it's not a good idea to cast all of your interior monologue in italics. Since long passages in italics (or, indeed, any unusual typeface) are a pain to read, you can only use this technique effectively for passages no longer than a short sentence or two. Even this brief passage is too long, don't you think?
***

Hope that helps.

You're totally fine. As Tuuli mentioned, it can be easy to overdo it, so be wary of that. I've found that the italics are great when I want to emphasize the thought. I save it for important stuff, and stick to regular internalization for the rest.

This is also a very Austen thing to do, which is never a bad thing.

If it were me, though, I wouldn't mess around with the italics. It just seems kind of comic book-y to me, but I may just be weird.