This sounds like a very difficult situation. You're getting used to a new country, a new job and have a large group of students to deal with. I have always found that very large groups are the most difficult to teach. There are very often different levels in the same class.
Here are some links to other posts to get you started.Teaching large classes for oral English in ChinaGroupwork and pairwork
As for mixed ability, there are two major opinions; the first is that you put the students into groups according to their level (strongest students together, weakest students together etc) and the other is that you organise group work with a mixture of stronger and weaker students in each group. I suggest you start by organising group work according to level. Give students the same work to do otherwise you’ll spend too much time preparing different exercises. You can give the same exercise to the stronger students and add tasks such as using 6 items of new vocabulary in their work, or using a certain tense (e.g. present perfect continuous) at least twice. The weaker students can do the basic task without the need to use complicated language structures.
As for your classes with 15 and 22 students, could you please send in more information. For example, are they children or adults? I would give different advice for different age groups. What level are those classes?
All the best,