I understand your situation, but you are the boss / office manager and sometimes you just have to act like a boss.
It's frustrating for other employees to see a certain employee "get away with murder" just because the boss is too chicken to step in and say "knock it off". This causes low employee morale. Why should I take some time and effort to look decent and professional when I go to work if my office mate can show up looking like a bum or a slut?
I really think a well-conceived dress code is good for business. It de-personalizes things. Rather than say "You look like a hooker in that tight t-shirt", you can say "Tight t-shirts are contrary to the dress code."
If you need a good contribution to the managers meeting, this might be a good place to start.
Employee position. People doing manual labor would have a different standard than those in the front office.
Male/Female differences: Yes, it's OK to have different requirements for men and women, just so long as you don't expect onerous compliance from one and not the other. You can, for example, require females to wear bras, but not males. We allow sleeveless shirts on females, but not males, so long as the female shaves underarm hair and only during hot summer months.
Piercings and tattoos: What best suits your business? Define acceptable and non-acceptable.
Weather conditions: Can females go bare-legged during hot summer months? Must they wear pantyhose with dresses & skirts during colder months?
Friday casual: Yes? No?
This is a short list. You can probably think of other issues that pertain to your business.
Once a proposed dress code has been drawn up and passed around the managers for comment, make it public. Pass it around the employees for comment, with a deadline. Then, take the employees' comments into consideration and prepare the official version.
Finally, encode the penalties for violation. What happens? Is the employee simply given a warning? How many warnings? Is the employee sent home? Can they come back that same day? Do they lose pay? Make sure that anything here has the full backing of management, and is in compliance with the law.
We had a funny thing happen when we passed around our proposed dress code for comment. We said "No tattoos". Someone anonymously said "What about the tattoo on my butt?".
Well... we are not in the Butt Inspection Biz, so we amended the dress code to say "No tattoos may be visible while wearing business clothing." Got a tattoo on your butt? We profoundly don't care.