I agree with your contractor. Different brands are fine but stick with stainless. Stainless finishes from brand to brand work well together. All the stainless appliances in our kitchen are different brands...no problem. Choose each stainless appliance for the features you like and don't worry about brands.

Agreed, buy what ever appliance brand(s) you prefer and have the features you desire. Matching is not important, getting what you like is.

None of mine are the same brand, but all are stainless. I enjoy every appliance for a reason, not because they came in a package deal.

Brands do not have to match. Some brands can produce a great fridge for example but the matching range functions poorly. Matching finishes can be generally important but there is exceptions for this as well.

You do not have to match brands, or handles. I always say - Quick, think of your friends’ and relatives’ kitchens. What do their appliances’ handles look like? Is all their stainless steel the same? Can you answer those questions?

Also guess what, stainless steel can vary even within the same brand. The manufacturer could buy their stainless steel from different sources, or of different runs, or their appliances are built in different factories. No one will notice if your appliances brands aren’t the same, if the handles are different, or if the stainless steel is shade off.

It's fine for the appliances to be from different brands in order to get the best blend that works for you. Like others said, be sure to go with stainless steel for consistency. For me personally, I prefer the handle styles to be somewhat similar. Since it's a new custom kitchen, you could use a panel ready dishwasher and/or built in fridge (not sure if you're doing this or a freestanding) so they blend in with cabinetry. Then it truly doesn't matter if they match or not. Or, if you go with a stainless steel dishwasher, you can get a pocket handle, so you don't have to worry about matching handle styles.

You didn't mention what style home/kitchen this is. If it is an ultra modern and sleek kitchen, then you may want to prioritize matching a bit in order to achieve that very specific look.