Each person has their own preferences when it comes to how they want to be held accountable in their relationships. The age play dynamic introduces a lot of opportunity to build a power exchange between a Big and a little... but ultimately, your list of baby rules should be decided by negotiating with your subby, and talking through the consequences.
I suggest you start by creating rules around how often your little one will be changed, who is allowed to change them, and under what circumstances. Second, you should have rules for who is responsible for disposal of the diapers and how often it should be done. Maybe the smell of lingering dirty diapers is part of the experience for a little, but if their partner would rather keep the house smelling fresh, this negotiation is where you talk out the consequences of this full time arrangement and make comprimises you can both agree to.
So here's the thing about rules... It's not so much about enforcing the rules, as it is setting expectations and creating rituals around the accepted use of diapers all the time. By creating these rules, you're saying two things to help you gain trust and obedience from your little one... 1) You accept and acknowledge their diapers, and 2) being in a committed relationship requires mutual accountability.
Not all the rules have to apply all the time, as you said, you and your little one will act differently when you're alone together than when you're in public. You should set clear expectations about things like public attire and behaviour, then make commitments not to cross the lines that are hard limits for you.
If you enjoy the role-playing aspect of diapers, try to think of punishments that can be self-enforced (I like to call them "funishments") and treat them differntly than the punishments you use to train their behavior. Funishments are basically scenes that put you in a little headspace deliberately for role-playing. For example, a lot of punishments might actually be a lot of fun for your little one, like writing lines ("Mommy is always right because I am just a diaper baby that always obeys Mommy." x50), or getting spanked, put in time-out, locking diaper covers etc... Find out which ones are a turn on, and which ones are truly a punishment. Mouth soaping, chastity, enemas, baby food... think of the toolbox you have at your disposal and how far you want to take the fantasy.
Here are a few examples of rules that we have at our house:
- Wet diapers have a special garbage can and get emptied at least every 3 days. Messy diapers get bagged and taken out immediately.
- Eating breakfast is important for little boys like me, so if I miss breakfast, I might find out that I have a big smoothie of spinach and peas and fruit juice that mommy makes me drink from a bottle. (This is borderline a funishment for me, because it's such a loving and positive way to reinforce my bad habit of skipping breakfast. Sometimes she'll juice garlic and put that in my bottle... then it's kinda more punishment, even though it's really healthy for you.)
- I always sleep with a bedwetter pad if I'm diapered. If I forget and leak, then I'm in charge of laundry for the whole week.
- If mommy cooks dinner, I'm responsible for the dishes. If I miss a night, I lose a privilege... like cartoons on the weekends.
We had a sticker chart that I designed at one point... got the funniest looks from the guy at Staples that helped me print it. Oh well, we filled it up and didn't make another one, but that's a fun way to take rules and create rituals and rewards.
Good luck to you! Rules and the accountability games in the context of age play is quite rewarding once you know what you want and what will work for your lifestyle.