When I first started incubating eggs in my Brinsea, I had a mix of "Kampung' (village) chicken eggs and some duck eggs (Large Mallard-Campbell/runner cross breed). Then a friend gave me a mass of quail eggs and as there was a bit of space left, I put 6 quail eggs in). The temperature was set to 37.5C which was inside the range for Quail, chickens and ducks (and guinea fowl in my book). The Quail all hatched but some got straddled and some just died and one drowned due to my incompetance/inexperience. The chickens had a good hatch rate and I got 2 ducks (from about 10 eggs)on day 28. The Auto turn was on the whole time (my first time at this!) and I don't think the quail liked being rolled around albiet slowly having just hatched. The brinsea floor is also plastic and a bit slippery. I now put a damp towel over it it add some friction for newly hatched chicks. I cant think of any reason why not to mix eggs as long as temperatures are within the same range. The problem comes when they hatch at different times. You can turn off the auto turn but then will need to manually turn the other eggs - should not be too difficult. Although I must say in my experience chickens and ducks are pretty tough! I cam back aftera week end away - auto turn on and found ducklings quite happy being tipped 45 degrees each way!!
You will pleased to hear I have improved my technique and try to be around during hatching - although I still get caught out when the blighters come out early!! (with me screaming "that's not what it says in the book"!!!!).
If it was me, I would give it a try. However, Guinea Fowl are more tricky than chickens and ducks (in my experience) and I would not wish to jeapodise them for the sake of the quail.
I did try putting quail eggs under a hen with some her eggs - and she ate them!!!
I have one pair of quail but the female just does not sit on the eggs. The male escaped a while ago - I found it two weeks later under a bush near the house. How it made it up there (over 100 metres of pretty exposed dry rocky sleeply sloping ground - not to mention cats and dogs! Anyway he was spotted and put back with the Mrs - He did not waste any time!!!!!!