They do all talk to each other, its can-bus aint it. The speedo uses info from the ABS computer, im not sure if you lose the standard ECU altogether what affect it will have on the ABS tho.
Im not sure on the clio, but on some models information from the brake pedal switch is used to cut engine revs ( for emmision ) So on some models removing the ECU could cause problems.
Dont take any of this as gospel as it could be b****cks, not worked on renaults for a long time.
Edit: Yours is a PH1 right? So speedo drive from box not ABS, 1 less problem to worry about.