hey there. I totally understand where you are coming from with this kind of question. I have owned a Zero for over 6 months & can honestly say that YES, THE SELECT & 'B' BUTTON ARE TOO CLOSE TOGETHER.
Having said that, there are alternatives while playing games that utilize multiple buttons during gameplay, etc. First, some games have the option to change the control scheme. Second, use muscle memory to adjust your RIGHT THUMB so it doesnt affect/touch the SELECT button repeatedly. I have big thumbs & I learned to adjust the way I press the 'B' button over time. It takes some getting used to but if you can let that pass & adjust quickly, it really isn't an issue.
Try to hold your right thumb in more of a horizontal way instead of diagonally & you should be absolutely fine. Again, I adjusted quickly & have no complaints. And also, the issue only becomes an issue when playing button-mashing games or fighting games. Other than that, I wouldn't worry about it too much.
Hope this helps.