As already pointed out, there are a number of fixed stalls and portable ones all over the city. From what I have seen they are made up of two types;
1. Fixed, a small booth the size a restroom stall or a little bigger. They normally have dry goods, snacks, pocket items etc. Some will have cold drinks, with and without the cold part. Many don't appear to have electricity, there are a few that do have power and will make photo copies of documents. Ones that do cooking mainly use a tank of gas with a simple grill setup, very simple as in half of a 55 gallon drum type, but it serves it's purpose.Â
2. Portable, these seem to appear daily in doorways, along side streets etc. The grill setup is as described above, and a cooler may or may not be used for meats. No long term storage or refrigeration seems to be in use. Also no sink etc. Sanitaton is a very big question and concern.
I have not seen any posted license, permits, inspection documents etc. You can also find mobile carts selling ice cream etc. The other thing is at many intersections you can find people selling fruit, water, small electronics, phone chargers, cables etc. I have no idea if this is controlled, it seems not to be. But a Gringo in a box would draw attention... It could be used to your benefit, as many people think Gringos do better business.
What kind of interaction you receive from other vendors would be hard to say. The normal perception is that Gringos are well off, so taking money from them may not be a welcome change. There are a great deal of under or un educated folks here, even though there are many small business people trying to make a buck. You may not be well received.
The goods sold are very inexpensive, so to compete you would need to be very creative in what you sell and how you sell it. You could make the Gringo thing work for you, but speaking spanish would be a must. You have to remember that you are in a place where you can get a meal for $2.00. Most customers seem to be people in transit, at bus stops and choke points throughout town.
It is doable, the type and quantity of hoops required is unknown. It would be an interesting experiment to conduct.
Good luck,