I have really enjoyed using DXM as a potentiator, although tbh it's can become something of a hit or miss thing. The standard suggestion for this is taking 60-90mg taken about 45 minutes before dosing the methadone, probably hedging toward the 60mg suggestion if you feel the dxm on its own. If I do this everyday, however, I think it builds up in my system, or as an NMDA antagonist it lowers my tolerance to itself, to the point I get fucked up from the DXM a little, even at said lowish doses. So, that said, I wouldn't recommend doing this every day.
Regarding the DXM, you're going to want to use any brand that only has DXM listed as an active ingredient. Many dm preparations also come with guaifenesin as well as DXM, and I've never had a problem with these. I'd suggest staying away from any other preparations though. I also find the generic store bands to be easier on the stomach and more palatable than the brand names, which are too thick and too nasty imho.
I've not tried hydroxyzine, but as an aside I find Cimetidine/Tagamet to be a great all around potentiator. Unless you have a serious tolerance, in which case DXM might be more useful, I find I get more out of using an antihistamine like diphen and ppi like cimetidine as potentiators rather than DXM alone or with anything else.
Look to the Opioid Potentiation megathread for more info // Don't mean to be a dick, but, as folks much more sagacious than I had said, "The search function is your friend."