The water shutoff valve doesn't change line water pressure, you can adjust the amount of water flowing through the valve like you did by turning its handle. Although I have no experience with this particular valve, I’d assume that it should be working fine with any type of a water shutoff valve; quarter turn, saddle, etc. However, some valves may have line water pressure limitations.
I’m not sure what type of adjustment there is on that float valve, but it says “Adjust float level after install” on the HD website, maybe you need to play with that adjustment to find a perfect spot.
See if you can close the water flow by manually lifting the float. If so, it should be the matter of fine adjustment to achieve complete closure. If you are unable to close it manually the valve could be malfunctioning, maybe it has a damaged seal.
Limiting the water flow by turning the shutoff valve to almost “closed” position will just cause the reservoir to fill-up slower. Also, in some instances it may prevent the floating valve from closing completely (depending on its design).
There are several float valves available, just type “float valve” in Google search window, click “Shopping”, and check the results. You may need to order online but make sure you pick something that will fit into the assembly of your humidifier unit.
I hope this helped a little, let me know if you have more questions.