In regards to use it or loose it for vaction/sick/personal time.

1st is it leagal to say you must use your vaction within a set time period or loose it?
If so is it legal to have some employees be able to accumulate their vacation and use it anytime within their employment (say as extra for maternity leave). While other employees fall under the use it or lose it policy, as long as you are not descriminating based on protected classes but rather on length of employment and/or position?

Also in regards to the employees that are allowed to accumulate their vacation time. If they have accumulated say. . 2 extra weeks of vacation time and they are fired or decided to quit do I have to pay them for the extra time they have accumulated?

Use-it-or-lose-it policies are not prohibited in Missouri.

Yes, as long as, as you state, the different treatment is not based on a legally-protected characteristic.

Missouri does not require unconditional payment of accrued vacation at termination. Generally speaking, the employer must comply with its own policy regarding this issue.