Pre-emergent stops sprouting seeds. It doesn't know the difference between weeds and ornamentals.

"I have already cleared all the plants that were the best that i could, although i am sure some roots/seeds still exist. "

OOPS! An herbicide can't work if there are no leaves to spray. Kill first, till later.

A pre-emergent would only work on seeds, so if you are planting the grasses as transplants from pots, you are OK.

Suggestion:

Water the patch thoroughly and let anything that wants to sprout pop up. Lure them out of the dirt.

Let them get a few inches high, then apply glyphosate (Walmart's Eliminator brand is a good concentrate) to kill them and the roots they are on.

Transplant your ornamental grass into where you want them, or plant the seeds.

You will probably have one bad crop of spring weeds to deal with. ... Be vigilant and hand pull them until the grass gets big enough to smother them.

As jean has already written a pre emergent will work. Corn Gluten Meal or the commercial pre emergent will both work. However, pre emergents will keep any seeds you plant from growing as well, but there is some evidence that plants growing from roots will not be affected since the pre emergent keeps the seeds from rooting.
Pre emergent also have a limited life, usually about 6 weeks, and may need re application.

Plant your new plants, pulling out any old roots as you go. Then MULCH. You will know that you don't want anything coming through the mulch, and you can pull them out as they surface (which the mulch will help to prevent).

I place my ornamental grasses in tubes of plastic (large diameter flower pots with bottoms cur off.) to prevent the roots from taking over the entire garden.

Tilling the soil activated the old seeds that were in the soil. All plants have multi-annual seeds. Some sprout this year, others sprout in the next few years....

If you are planting specifics, why not put down 3 layers of newspaper with 2"-3" of cooked hay.

DO NOT USE "landscapers fabric". It kills worms and other beneficial insects.

Wood chip mulch is almost as bad.

I must disagree with the last poster on the use of wood chip mulch, as I have lots of happy worms under the mulch on my garden beds.

If the OP needs advice on mulch choices, I advise a visit to the Soil, Mulch & Compost forum. (hmmm - might be Soil, Compost & Mulch).

Granted, it is plenty good for happy worms but that which breaks down wood chips also takes up nutrients before the plants get them. You are actually robbing your soil of food that the plants need. I guess you can overcome that with a lot of fertilizers though....

Not so happy for beneficial insects: Shall we talk about all the chemicals used to get those consistent colors? How about all the insecticides used to kill off the insects in the original trees? Just think of all those built in herbicides and insecticides. Did I just hear a groan from Rachel Carlson?

The matting, that wood chips make as they break down, destroys the hives of the bees and other pollinators that nest in the soil.

Then, of course, the manufacturers do not use scrap wood chips or second source materials either. They are chopping down trees to make the mulch. This is not unlike the destruction of swamp bogs to make peat moss and peat pots.

It is actually much safer to use Round up to control weeds if it is used correctly. Granted, I don't use it myself as I use other methods.

There was just a series of posts about the now-disproven theory that wood chips remove nutrients from the soil - you should come over to the forum and read.

For what it's worth, I do not use dyed mulch, and I think that you are protesting just a bit too much about the trees from which the wood chips come. I invite you to stop by the SCM forum and make your points there.

Granted, this is becoming a bit off topic as this is much bigger than herbicides...

SCM forum?
I guess I will have to look around for it....

I found what might be what you are discussing. Soil, Compost, Mulch forum. I saw little about wood mulch good or bad.

Do you have any URLs to offer?

Wood chips when used as a mulch, and not a soil amendment, do not create problems in the soil but are quite beneficial because they help conserve soil moisture, help suppress unwanted plant growth, help keep soil temperatures cooler, and will over time provide the soil with organic matter.
I have never seen anything that would lead me to believe that wood chip mulches are harmful to ground nesting bees and have had them nesting in the ground under wood chip mulches around here. If there are any articles that support that please refer them here.
While out west and down south there are companies that do clear cut and chip up the unwanted trees for mulches (cedar and cypress) many also harvest trees from tree farms where trees are grown just like other crops and as soon as the mature trees are harvested new trees are planted, think Christmas tree farms.
Glyphosate products present a very large environmental hazard, much more then wood chips used for mulch ever will be.