It is rather difficult to find yoga classes in English in St. Gallen...
However, in Sundari Yoga (www.sundari-yoga.com
) the teachers can speak English (I am doing my Yoga Teacher Training there) plus Chiara Castellan and Katchie Ananda are frequently invited for workshop in English.
Otherwise you can wait until I get my Yoga certificate in October :-D
For the best area where to live in, it really depends on what you are looking for.. Any part is generally well connected to the city center, where most of the shops are. I live in Riethüsli and this area (and also St. Georgen) are pretty quiet, with nice views and close to nature.
Hopes this helps!