Is there a chance a aero mod hurts your mpg
 

I install rear wheel skirts and ground skirts and a rear spoiler. Is there a chance I've created more drag? What if I seldom drive over 50 mph. Could the front skirt and grill block possibly create more drag at low speeds? Sc

It mostly all depends on the specific vehicle. If you could post some pictures then it could make it much easier to take an educated guess at it. As far as rear spoilers, some help, some hurt mpg, again it all comes down to what spoiler on what car.

I'm guessing that as long as your rear wheel skirts don't add a ton of frontal area, they will be beneficial.

more drag?
 

For the mods you've mentioned,the rear spoiler would be the biggest candidate for a potential drag increase.The spoiler's upward inclination angle would be the most critical factor.
As far as speed goes,if a mod is providing a drag reducing function,it's going to be that way over the entire speed range of the vehicle.
At 50 mph,your rolling resistance would almost be as critical to mpg as aero.At higher velocities the aero drag would dominate.

Normally spoilers are a bad thing for aero. We decided that unless your spoiler is sitting pretty flat against the trunk lid remove it. Now spoiler is not to be confused with kammbachs a good amount of people get spoilers and kammbachs confused on for example suvs there called spoilers but in reality they are kamms

Ok here are the pics. The front skirt is bending under the car. Bad?
The rear wheel covers, Seem ok but a racer guy said it creates turbulance, The spoiler, x-cookie pan, looks like its ok, or is it? SC

The air dam looks fine

The rear wheel skirt looks like it could be more flush with the body of the car.

As far as your "spoiler" i would slope it downwards slightly to make it more of a kammbach, from the view from the picture it just looks like you extended the top to be even longer, a slight downward slope will allow for airflow to be attached partially downwards forming the airflow behind the car.

an other issue is I scored those low profile tires and rims at the pull a part for 30 bucks each, would you think them awesome rims an tires are laying a hurtin on my mpg? Sc

it depends I believe i read somewhere that the lower profile tires make the tire heavier more metal less rubber/air, but has been said that if you do strictly highway can be better for more rolling momentum (harder to get moving). I personally stay away from them because if you get low profiled tires that adds to the price of the tire

As far as the air damn and wheel skirts I would bet those are improvements ober the factory setup, the rear spoiler, as Gealli said it would be more beneficial if you could slope it downwards a bit.

Perhaps someone should let him know what angle (in degrees) he should bent it to?

I would, but I don't know the exact numbers ;)