I think it's common for people to give an opinion. Not that it's right but it just happens a lot. Which is why it's best to do what you want with your wedding and what you can afford, because you can never please everyone, there's always gonna be someone unhappy with your choice. The only exception I'd say is if the parents for example are paying for all or majority of the wedding, in which case it makes sense to me that they would have a say.
In terms of plus ones it's always a hard one cause not everyone wants random people at their wedding and the associated cost. I know people who say at their wedding no one will have plus one unless they know each other well and have some kind of relationship with that person, but wouldn't invite eg their friends husband or wife if they are not friends with them too. Which makes sense to me but on the other hand, I probably wouldn't go to someone's wedding if I can't take my partner, unless I know lots of people there