Of course you can. If you want to sell different products in the same niche, then building a site that compares them works good.
You can work with as many a you like. Since the brand dictates the program you may have to work with many networks to build out a shopping or coupon site.
I agree that most affiliates promote more than one product. The principle of "not putting all your eggs in one basket" and all. What if the company you're promoting goes belly-up tomorrow? You need to have a back up plan.
Yes, there are always different ways of promoting them and new niches out there that not always one place can cover.
The 2nd line of my sig combines 6 of the finest affiliate programs together and will set a website up for you for free. Earning multiple streams of income is the best way to make a lot of money online.
Advertisers can work with several affiliate programs. What will you get if you join only one affiliate program? A little bit.
I have several that I use..many of them have the same offers, but payouts are different and it helps to find out who pays, who doesn't, customer service, etc.
Of course you can use as many as you want, but it is good to know who is a reliable network and who offers what you are looking for.