You are coming to the city where I live!! Isn't it great? Regarding your question. It's perfectly right to wear jeans. The temperature will be between 10 and 20ºC (Celsius grades not Farenhait, don't panick!) It's fall here. So you might get hot with jeans. I recommend you to use the onion tecnique. A shirt and a coat and may be a short sleeve t-shirt under the shirt. If you feel cold, you can use all of it. If you feel hot, then you can use only the t-shirt. Concerning appropriateness, uruguayan dress very informally, so don't worry.
By the way, when are you coming? Do you speak Spanish? How long are you staying? I would like to know you!