Yes you can mix them in the same syringe as they're both oil based.
As for needles, you might find it takes a long time to get the oil through a 25 gauge needle, I use a 23 gauge 1 inch for quad injections, comtinue to use quads if thats what is comfortable, use both legs and rotate the sites, you can do the out side of the quad and the front of the quad so 4 different sites on 2 legs. check out
www.spotinjections.com and you'll see what I mean.
I never had a problem just doing it in my quads.