Do flippers get hit with short term capital gains since they buy and sell within one year? How would this be avoided?

No, the proceeds earned from flipping is income, not capital gains.

The properties you acquire as a flipper are your inventory, they are not capital assets used to earn income.

That is good to hear. But does it make a difference that typically, we hold our properties as rentals, and only flip occasionally?

Tom:

Your intention when the property is purchased comes into play.   If you typically fix and hold, but are occasionally opportunistic you may be able to make the case your intention was to retain the property and rent it out.   However, if this happens with regularity, the taxman could unilaterally decide you are in the business of flipping and your proceeds would be taxed as income rather than as a capital gain.

@Roy N. hit the nail on the head. If you did a flip, it is ordinary income. If it was a potential rental turned into a flip, hopefully you have documentation. This might be MLS listings for lease, other advertisements for lease, etc. You can do both flips and rentals in your business, but the properties are treated differently.