would this work for a summer stable sheet???

Possibly the higher end ones are higher percentages of cotton - the last Kool Coat I purchased was about 7 years ago and it had a very high percentage of polyester, my horse sweated up a storm.
If there is a higher percentage of cotton, then it will stay cool. I might have to suss out the new Kool Coats and see if they're better now, as they're good fitting rugs.

Get a fly sheet if anything!! You don't want to put an actual blank or rain sheet type sheet on as it will be too warm!!

Meh, probably wouldn't work.

I wouldn't. The only time I will cover my mare in the summer is for a show.

And even then, I'm there until 8 or 9 pm finishing up, and only put the blanket on once she's in the stall. Then get there early in the morning and take it off.

She's a bay, so I'm not too concerned with bleaching.

Definitely just get a mesh sheet. If you're covering just to keep your horse clean, then there's no point. It's dry in the summer, so they're not rolling in mud or anything

That looks like it would be too heavy and not breath, although the color is fun! Its not worth it to have a sweaty unhappy horse. I would go for light colored or white mesh flysheet!

I was talking about for in the summer like before show, I usually wash him at home the day before show and put him in the barn by he has a habit of laying down in the stall at home. I have a sleazy to put on him the morning of the show but my 4h leader doesnt want me to put it on him while he's at home in the stall incase he goes to the bathroom then lays down they want me to put a waterproof sheet on him so it doesnt soak through the slinky it would only be on him for the night before show in the stall at home

Still not a good idea. Especially in a barn, there isn't even a breeze to help. It isn't breathable, and if he sweats (which is inevitable...not only with heat, but with the stress that horses get from being stalled), he'll be very uncomfortable. I have never had an issue with a stall mess leaking through a sheet.
If he lies down, he lies down. There are ways of cleaning up stains on his coat, without re-bathing, and without making him suffer through the night with a sheet that is too hot.

Still no.
If he lays down in his manure/urine then tough luck - you'll just have to get a damp sponge and brush off stable stains in the morning.
I compete quite regularly in official dressage competition, and no matter how big the show or how early I have to leave, I will only rug to the conditions. I'd rather have to brush off a couple of marks on my horse's coat than have it sweating and getting stressed overnight because it is in a rug that is too hot for the conditions.

Use extra bedding!
Get fine bedding for underneath to suck up the pee and then thicker bedding over it.
That's what I do.
I also put her in a stall closed to the outside of the stable.
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