You certainly won't bulk up from strength training, but you may find that the scale will show a temporary "gain" of 2 to 3 pounds for a few days as your muscles hold in more water. It will flush out, I promise!! I think it would be great to do that!
If you have pain, actual pain not just soreness or slight discomfort, then, yes, avoid it. I've found that sometimes my heart and lungs can handle the cardio, but it takes awhile for my tendons and ligaments to strengthen and I end up with pain if I increase time/intensity too quickly. YES - as you lose weight it gets MUCH easier!! I notice a difference in just 10lbs. The pressure on my hips and knees drops a lot. I think the rule is something like don't increase more than 20% per week? Walking is great for you. You could try the elliptical and see if you like that. I am not sure that I would wait to increase your frequency until you lose more weight, because I think you'll get more results from frequent low intensity than you will medium intensity just a time or two a week.
Maybe don't put so many rules around it - just walk more often and add the strength training and see how you feel. You can always change it.
HW: 225, lost 75lbs in 2011 LW: 150
Losing again starting from 205lbs - 7/31/2016
No words of widsom. After a lifetime of reflection and dieting, I still don't have this thing figured out. I have decided, though, that sugar is the devil and I'm cutting it out as much as I can.