I have several spare 240 breakers in my panel box. I'm adding 8 outlets from 2 different rooms. Can I use half of the 240 breaker instead of replacing with a 120, or should I pull the 240 and replace with a 120? One of the spare 240's has wires coming out but are no longer in use (for old baseboard heat).
Yes. The only issue you'll get into is if your nation's electrical code requires a higher grade of breaker. For instance most breakers in the USA or Canada need to be AFCI type. You'd be better off with single AFCIs rather than fooling around with a 2-pole AFCI.
In some cases, both AFCI and GFCI is required (or desirable) and in that case an AFCI+GFCI dual-mode breaker is cheaper than an AFCI breaker + GFCI recep(s).
If AFCI is required you cannot use an AFCI recep unless you run metal conduit.