Correct me if I am wrong but I was always under the impression that it was not a good idea to mix CNS depressants with CNS stimulants?
As far as pseudo goes, I think I've read that anything over a 100mg can cause an overdose in a non-tolerant user. Obviously, the 120mg tabs you get from the chemist are slow release and do not allow the pseudo to go into your blood stream directly.
I am not sure if pseudo is cross-tolerant with meth, but I find I don't feel even the slightest buz when taking a few sudafeds these days. This would lead me to believe that if someone is fairly tolerant to meth, they can feel pretty safe with pseudo, unless of course you bang half a gram of it.