I did this with my nano CPX for a short while (maybe two flights). I don't recommend it. Main reasons (both assuming a setup with a happy medium pitch setting through the entire throttle range, where head speed is not bogging or overly high in hover):
1) You will learn bad throttle management.
2) Cyclic response while in descent will be worse on a pseudo-FP setup CP heli, than on either a real FP heli or a normally setup CP heli, for most reasonable rates of descent (ie. not accounting for the ridiculous rates of descent possible with negative pitch on a CP heli).
Details on #1: Suffice it to say, a pseudo-FP setup on a CP heli will result in a tendency to climb very easily from hover and overly lag throttle input to fall from hover. What's more, it will be very sluggish to climb from an overly aggressive downward elevation maneuver (due to having to overcome an overly low head speed) and very sluggish to fall from an overly aggressive climb maneuver (due to having to wait for the high head speed to bleed off). This will likely either train you to be too slow in your throttle management (to avoid overshooting) or too aggressive in your throttle management (by purposely feeding impule or overshoot inputs into it and quickly backing off). And if you learn to get it just right on this pseudo-FP setup, you will still find you are at a worse place when it comes time to transition to full CP settings than you would have been using a real FP heli because your throttle inputs will be exaggerated compared to a normal FP heli, which will result in a steeper throttle input learning curve on a CP heli (or at least an abnormal pitch curve that takes this into account).
Details on #2: As head speed lowers and lift efficiency falls, cyclic response will also fall. On a FP heli this is minimized by having higher efficiency blades, which will maintain cyclic response though a broader range of lower head speeds and resulting rates of descent. Alternately, on a CP heli, cyclic control is maintained even when the heli is descending on medium throttle as a result of maintaining a relatively unchanged head speed by lowering blade collective pitch. However, in a pseudo-FP setup on a CP heli, this will not be the case. Collective pitch will be held constant through the throttle range. So the resulting significantly reduced head speed at medium throttle will result in some loss of cyclic control compared to both the FP and CP at a given rate of descent.
If you want to fly FP, get a FP heli. There are many and they are relatively cheap. Transitioning from FP to CP is hard enough, don't put yourself at the added disadvantage of learning pseudo-FP flight on a CP heli.
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Oops, cut out that line. Just realized you said you have a 120SR and mSRX already.
Stick with them. Go ahead and get a CP heli too, if you're feeling comfortable with your FP skills. The nano CPX is very tough and a lot of fun and great for indoor flying and light breeze outdoor flying. I'd highly recommend getting it with a programmable transmitter if you don't already have one, so you can program in some D/R and expo to tame it down. It is highly reactive compared to the 120SR, which is what I started out on. I used much more D/R and expo than the manual specified to start with: +/-50% as my D/R maximums for A/E/R with 30% expo, to start with. Made it manageable. Up to 75% of maximums now with 20% expo. Don't know if I'll get up to 100%/0% until I get into some beginning 3D stuff.
