I would caution anyone with pets about bringing xylitol into their house. Reason being, it is a deadly poison to both cats and dogs. So if you have critters, and want to use xylitol for sweetening sodas or anything else, PLEASE do it in a place/way where curious critters can't get into it...
I mostly use Stevia to sweeten my diet/low cal sodas, but if you want to ise aspartame/ace k, you would have to do all the heating first (steeping flavoring ingredients, etc.) then let it cool completely before adding your sweetener. You could also try sucralose (Splenda), as it is not supposed...