The problem is not the ratio. The problem is the number of teeth on the pinion. When you get low tooth counts, the gears tend to become more undercut and weaker. Achieving contact ratios above 1.0 under all tolerance conditons may prove to be problematic.
My recomendation is to keep your pinion tooth counts high enough to avoid these types of problems. The actual tooth count that you can get depends on the diametral pitch, pressure angle, mating gear number of teeth, loading, and quite frankly how good a designer you are.
Generally with 20 degree pressure angle systems, I recomend novice designers to start with a minimum of 16 teeth on the pinion and work your way down as your can develop your design. In most cases I can accomodate 12 teeth in my designs. You may get to 10 depending on the diametral pitch of the system. If you need to go lower - consider helical gears to increase contact ratio.