That is a good question! My boys never liked yogurt but they loved baby food (i.e. peas, squash, carrot, etc) and those were just strained veggies essentially, so I did not have to worry about the sugar content, but I am not sure how much sugar is in strawberry yogurt. Anyone know how much is too much?
You shouldn't give too much yogurt, because many rats are actually quite sensitive to lactose and dairy products. I only give my boys yogurt if they need to take some medicine (don't mix it with doxycycline, but other meds are usually fine with it). Otherwise, it's a rare treat, and even then, you shouldn't feed more than a tiny bit (e.g. fingertip amount) at a time.
I've given my boys soy yogurt because of the lactose intolerance. There's a brand called O'Soy put out by O'Baby.
Now some will tell you not to give them soy or not to give it to them after a certain age because of fat content and some kind of cancer link, but I've heard conflicting opinions on that. I gave my youngsters soy yogurt about three times a week, no more than a tablespoon each. I give my adult boys soy yogurt as a treat every couple of weeks, roughly the same amount. But I'm no rat dietician. That just seems like a sensible plan to follow.