Yes- I  wonder about that, as well . I seem to have a running internal dialogue constantly , speaking to God as a confident and a friend, but it is not structured or worthy of His Greatness. I must admit, many times it's complaining about things!
I wonder if I should train myself to stop being so casual with Him and limit myself to more formal prayer ( I do that as well)

You could always take up the venting with your Guardian Angel or one of the Saints. Listen for that little voice that benevolently checks these tendencies. That way, you might get some spiritual feedback that puts you in a better frame of mind to converse with God directly.

I even talk to the Saints this way. I talk with Little Flower all the time. Sometimes I'll just talk to Our Mother. We don't practice our religion with our feelings but at times when I am in my feelings I will confide in God, Our Mother, or the Saints the way I would with any of my closest friends. They are there to console us. I don't take it lightly but I personally do this to work on getting more comfortable with prayer because I really worry that I'm bothering God. I think a lot of this stems from the contentious relationship I have with my parents and how my needs were scarcely met and always waved off as bothersome by parental authority figures. I brought this up to my priest and he assured me that I will never bother God and that all throughout the Bible He tells us to converse with Him.

Thank you, Yeti!

Are we allowed to talk with God in a casual way? Does it count as praying?

go on a whole emotional rant like "I'm so mad this is unbelievable those f***ing stupid people" etc.

Any person (especially a woman) who talks like that should have their mouth washed out with soap! 

As for the topic, of course you *should* talk to God in a familiar way.

Are we allowed to talk with God in a casual way? Does it count as praying?

Yes.  Yes.  Arguably this is better prayer than just reciting formulae, since prayer of its essence is communication with God.

... it shouldn't be TOO casual, though, as if you were talking to some beer buddy.  You do have to remember that He is God.  You wouldn't think the equivalent of, "Yo, God, how's it hangin' today."

St. Alphonsus wrote a book on this subject that you would probably find helpful. It's called How to Converse Continually and Familiarly with God. You can buy copies of it on the internet, or even read it for free on Archive if you create a free account to log in with.

Ah, sorry, you wrote this before I got down to your response.  It's a great little booklet.