Not only is it okay to mix them, but it's perfectly natural. Arabs bring in influences from their dialect when speaking in fusHa. The extent with which this happens depends on the level of formality. You're only expected to speak in pure fusHa in very formal situations, which you're unlikely to encounter as a beginner.

I would treat this the same way teaching register is treated in English and focus on it with more advanced students. At that point, you would've acquired enough competence in the language to tell them apart.

I reckon that people who want to learn a certain dialect ,they only need that for a certain purpose and not for exploring the whole Arabic culture. Overall, peole have the right to choose what they want to learn. For myself I always recommend people to learn Fusha, because when you learn Fusha, it would be easy for you to talk with all Arabic people regardless of their dialect and sure they will respond to you in Fusha because they all know how to speak Fusha unless they are not Arabs.

For non native Arabic speakers, learning both MSA and a certain Arabic dialect at the same time, it's something almost like learning two different languages. Overall ,I don't recommend you to do so unless you have to do it. When you finish learning MSA , you may try to learn a certain Arabic dialect.

It's Ok to mix between them , some modern Arabic speakers mix between them to show that they're intellectual and know how to speak in MSA .

So I think you should focus more on grammar in order to avoid mixing .otherwise it's ok we can understand

Kayla:

I highly advice avoiding opinions based on absolutes that don’t take into consideration why you’re learning Arabic, such as that you must learn fusHa or you must learn X dialect. There is no absolute answer to this question.

Feel free to message me if you’d like help with deciding whether to learn a dialect and which dialect to learn. There are bound to be different opinions, but as a teacher I’d like to think that I offer an unbiased opinion based on each learner’s needs.

@Abdalá

I actually saw your post when I was deciding whether to learn MSA or dialect. There's so many differing opinions on what's best though, so I still don't know what I'll do.

I do prefer to learn dialect, but want to learn MSA, too. I think that if I learn MSA, it'll be easier to learn multiple dialects later because I'll see how they all relate to each other. Also my teachers taught me MSA and dialect at the same time, so if I don't continue learning both, I'll feel like something is missing. You know?