You can definalty do that with no issue. Allot of guys will leave their plasma set at the same amps for everything and adjust the speed accordingly.
If you look at the PW cut chart for 10 ga. (closest available thickness on PW cut chart to 1/8" which you said you would be cutting allot) you will notice the suggested amps is 40 which you could use the 0.9 tip for. The orifice of the 40 amp tip is .035" vs 0.044" for the 60 amp tip. Not a huge difference but there will be times that the smaller orifice will help you with more intricate designs.
This is a good reference I have found helpful posted by @mechanic416 a while back.
Maybe this will help you.
This is what I have found to work with most plasma cutter nozzles/tips that most people have in their shops or garages. NOTE: Orifice hole size 1mm = .03937"
20 amp tip - 0.6mm/.025" - 15 to 20 amps - 50 to 55 psi
30 amp tip - 0.8mm/.030" - 21 to 30 amps - 55 to 60 psi
40 amp tip - 0.9mm/.035" - 31 to 40 amps - 65 to 70 psi
50 amp tip - 1.0mm/.040" - 41 to 50 amps - 65 to 75 psi
60 amp tip - 1.1mm/.044" - 51 to 60 amps - 65 to 75 psi
70 amp tip - 1.2mm/.047" - 61 to 70 amps - 75 to 80 psi
80 amp tip - 1.3mm/.051" - 71 to 80 amps - 75 to 80 psi