I had a double gang box, that I only replaced one switch. I continued to use the existing double cover that was already on the box. I use GE Z-Wave Wireless Lighting Control Smart Toggle Switch, In-Wall, White, 12727 in all my non-dimmer replacements
In general, yes, they can be used in a double gang box. Most just take standard switch covers although some designer models, like Cooper, do make their own. Is also coming double and triple gang versions.
I am quadriparetic, so reliability is a big issue for me. I will pay more for a device which has a longer MFOP ( maintenance free operating period) because I can’t do the maintenance myself. So I pay a lot of attention to that particular characteristic.
There have been some complaints about the DragonTech wall switches (not the pocket sockets, which are made by a different manufacturer).
GE switches have a known issue where a relatively high percentage, maybe 10 or 15%, will stop working about six months after the end of the warranty period. They are not technically defective because they’re already out of warranty. But it can get annoying. Nothing to do with SmartThings, you’ll find this discussion on other forums as well. A lot of people are OK with that because the switches tend to be much less expensive than the other brands so they figure even if they have to replace 15% every couple of years they still save money on the overall project. For people who don’t want to have to do that kind of maintenance, other brands may be better.
In terms of overall reliability, both Cooper and Leviton have very good reputations ( and typically longer warranty periods than the other brands), but they are also significantly more expensive.
So as with pretty much any home improvement product there tends to be a good/better/best breakdown, and different models will fall into different tiers.
The following thread discusses some of the different features you might get from a switch which can help in selection as well. As always, different things work for different people.