No they don't need to be the same. Often people will match fridge and dishwasher as different companies have slightly different finishes for their stainless, and those two appliances have the largest surface area which might show the difference. A cooktop is at a different angle and will probably be fine.

No need to use the same appliances at all. Just be aware of the different appearance of stainless between manufacturers and chose a brand that has a similar finish to your existing appliances.

It doesn't have to be the same, as long as you choose the appliance that best suits your needs:
*Price
*Performance
*Style

Almost all of us mix appliances here and all but if they are transformers, then that's probably a big yes xD I'm just a kid but I sure know a lot of things ;)