Hacksawing off the shifter portion of integrated MTB shift/brake levers is totally fine, as long as you are not removing the clamp ot other parts necessary for the brake lever to be safely mounted. In general, this is usually possible, but there may be certain configurations or models where it is more difficult or a bad idea.

I did this mod for several customers BITD and it worked fine. The one thing that might be an issue is that the clamp remaining for the brake lever might be too wide and interfere with the separate shifters. I have never tried to narrow the clamp down because new brake levers are cheap and time is valuable.

Another option is to remove the separate shifters from their handlebar mounting clamp and see if they will bolt up on to the place where the shifters from the integrated units mount. If you have already started hacksawing then this might not be an option.