Do you put “rules” on yourself?

I have one rule. When a sim is born and ages up and I jump into cas for a makeover, I won't change any of their features! I will even keep the same hair color unless it fits the story of the specific sim. If I think their physical appearance needs to be changed then I will have them work out or eat more during gameplay.

I have a large save. I started with 8 "Original Sims" (OS) in Newcrest. They are the "Founders". And their children/grandchildren etc are "Founders Heirs". As the game progressed I added other fams, but they are not Founders. One rep from each of the 8 Founding fams is on a council (group) and they vote and implement rules (imagination!). For instance, an OS is not allowed to marry another OS. Only Sims "born in world" can move to another city. No structures over two levels high may be constructed in Sulani. Certain careers require a distinguished degree. There are more. It seems like a lot, but it really adds depth to the game. Each time EA adds a new town, I get to add a new OS. I've had the save since launch day. All of the Original Founders are dead and in the summer we have Founders Day. Fireworks, bbq, swimming, it's pretty great.

I love varieties, uniques, themes & weirdness in my games.

CC. Skins and overlays are to be avoided, period. They have been nothing but a hassle to deal with. Between getting broken, and the fact that I've had a ton of weird problems with it (i.e. Sims entirely missing their face, tongues and teeth that are floating in front of the head). I have (from places other than the gallery) downloaded Sims that originally used skins and overlays, just adding their Tray files and not the CC; and have been quite surprised at how good some look even without all that extra stuff. Of course, almost all of them are bald and wearing nothing, so you've got to start over with what the game has, not the CC used by the creator.

Mods. No mods that allow a Sim to do something I wouldn't. And yes, there are some weird mods out there. I honestly have no problem with the infamous, but widely used here mod that we can't mention by name, but there are some that involve some really sick, graphic stuff.

Actual gameplay. Unless my Sim knows another Sim well, I do not even bother to respond to their invitations.

I start with one Sim, let them fall in love and raise a family. I follow their whims for kids and stuff. Then, when I the kids age, they get the first trait that gets randomized. I don't alter the kids' appearances. If they're ugly, oh well. Then, I play all the kids and their families once they move out. It's a great deal of fun because each house always has something different going on. I also use no cheats.

No rules for now because I haven't started any challenges. I do have a 'never use money cheat' rule, but meh it's not a gameplay rule.

The only rule I follow is, that every sim has his own unique looking room. Even when I re-use some of my favourite rooms in different saves, I change the appearance so far, that it is still looking unique, but this room will never exist twice in one save.

I don't have many rules in The Sims 4. I'm still a frequent Sims 2 player and that one is full of rules for me. I did take over some. I don't alter my sims appearances. I don't touch their facial features at all and mostly I let them keep whatever they grow up in since in my mind they 'chose' the outfits (and hairstyle, if the colour matches what it was before) themselves.

I have a few main rules:

1. No cheats, except if I need it to fix a bug, etc.
2. No changing physical appearance of a Sim.
3. No creating spouses yourself, they must be a game generated.
4. All traits must be randomized.
5. No making money by selling gems, rocks, fossils, etc. (makes the game too easy).
6. Sims can move, but the lot must be 64 x 64 in size.
7. When moving, only collectibles, objects gained from careers or created by your Sims (e.g. paintings) can be taken with you.

My few rules would be how I do the university rotationally.
When I first enroll my sims to university, they have to go through 3 classes. Then they reenroll into 1 class and I rotate away for a week or more. When I come back, they'll have to do 4 classes. After that, 1 and I rotate away. When I rotate back, they finally have to go through 3 (or 4) courses.