This will rarely work if you are using "co-founder" as a way to get a higher dollar (caliber) employee for a cost which you can afford today. Co-founders are either all in and you have to forget what you have already done or you have to admit they are not a co-founder and instead are a critical employee hire.
There is nothing wrong with offering a lower than market salary and giving the employee a 5-10% ownership with vesting. That is completely legit.
I think the main problem is don't call someone a co-founder and mislead yourself or them if they will not be splitting decisions and the hard times with you. A true co-founder may be taking a salary when times are good, but has to be willing to forego it when times are tight (bootstraped especially). If not, they are not a co-founder they are an employee.
As far as an equity split, I do not feel every co-founder deserves an equal share, but I would always start there and then see what other facts might affect it. For example, 15 years ago in a startup I was an early contractor for, a group of people started the company but one of the co-founders had family to support. So he kept his day job quite a bit longer until the company was more solid and could afford his monthly minimum without struggling. They all agreed that meant he wasn't going to get the equal split, and I remember even he felt that was fair and they were great people to work for/with. And no one was trying to be an asshole to him, just he didn't have the same risk and skin in the game.