Soo I been thinkin of horses all the time,  dreampt I had 2 of em xD yesterday,  soo were there any slingers that shot from horseback?

Unlikely. If you can afford a horse then you can surely afford a bow as well. And this is assuming your horse won't throw you off with your first throw.

Hadrian's speech to the army in Africa (the "Lambaesis speech") mentions the equites Coh. VI Commagenorum:
 
Addidistis ut et lapides fundis mitterestis et missilibus confligeretis
 
-- Moreover, you shot stones from slings and fought with darts (translation M. P. Speidel, modified).  
 
This, by the way, is the only known reference to slings used by auxiliaries.  
 
Arrian, Taktika, 43.1, mentions, among the exercizes, the "special shooting" using darts, cross-bows, stones by hand, and stones by sling (so that the Kommagenian auxiliaries doing the sling thing isn't special, but routine in Roman army).  
 
[spelling corrected]

Seems very unlikely considering how the sling really needs the full body to throw well. I spent some time awhile back slinging from a kneeling position just to see what it felt like and it's not easy only using your upper body.  Needless to say it's not quite as effective.

Quote from Thearos on Mar 22^nd, 2012, 9:32am:

 
Perhaps I should spell this out: there is unequivocal evidence for Roman cavalrymen training in the use of the sling as part of their exercises.  

Seein as I may be workin with horses,  probably gonna get a saddle case for my 30-30 for bear protection since im gonna be sleeping in a tent for the summer xD,  slings for off the saddle no use taking a risk for someones horse I dont own  ,  maybe I can bring one of my bows for when I'm not working or try to shoot from the horse if its alright with them.

yeppers

It's definitely possible with a short sling and some kind of underhand or high helicopter but as I said when there were horses and slings the slings were being used against the horses, not while riding the horses.
 
Little definitely bring some kind of bow. Howard Hill even used a long bow on horse back to take down a buffalo but a bow around 50" would be a lot easier to use.  
 The skill of horse back archers is astounding, the Comanches could not only take lot's of shots on horse back but while riding bareback they could lean over the side of the horse, while it was running, and using only leg musles hold themselves up and use the horse as protection as they shot their bows from underneath the horses neck and then after shooting they could pull themselves up with just their legs and keep shooting. You alsohave enough time to make a real plains bow and arrows between now and summer. And you can just ask questions on here and myself and others would be happy to answer.