DXM is definitely a DANGEROUS idea with an SSRI like Zoloft, and many can testify to that. Diphenhydramine, I'd think okay to take in normal therapeutic doses (25 mg - 50 mg) even if on Zoloft, but am NOT sure 100 %. I'd suggest no codeine, or if you want, codeine. SSRI's tend to blunt the effects of opiates a good deal for a lot of people - if you already need 700 mg codeine, I would be worried about going much higher in that, and you might not feel enough for your taste at 700 mg. Even that is a lot of codeine, and I do hope you are doing a CWE (cold water extraction) and not consuming loads of pills with APAP, ASA, or an NSAID of some sort - that can lead to serious issues. Just be careful, and make sure you are being safe with that. Really, though, AVOID DXM at all costs when on Zoloft. As for mixing diphenhydramine and DXM, if you were not taking Zoloft, I would still advise against it, esp. higher than normal therapeutic doses of diphenhydramine, but still diphenhydramine has a more narrow therapeutic index than a lot seem to think - don't fuck with diphenhydramine unsafely or in high doses. But, no I would not mix DXM with diphenhydramine under any circumstances.