As Mike pointed out, this is pretty subjective and I think you can pretty easily make either argument. I think I'd tend to avoid the slider as it takes up a lot of space, but that's just an opinion. If you really want to find out if one is better than the other for conversion, test it using software such as optimizely! Otherwise, I'd go with your gut.
To answer your questions:
1. I think the practice you described sounds good, should help establish credibility and trust, right?
2. Using the SEM Rush example, I might add a quote from one of the above clients for added credibility. On your /clients page, I'd probably do a couple things: I would add quotes/testimonials for each logo if possible - it's one thing to work with someone, but it means a lot more if you have a quote from them. Second, I'd look at doing case studies if possible.
As I mentioned earlier, you should really test everything. Everyone has an opinion about CRO that's based on their experience but every vertical, niche, and company is different as are their customers so don't just take someone else's word for it. Test everything!
