Yes, you can (if implemented, of course: a few kinds of calculations are not available with USPP, a few more are not for PAW). A small restrictions exists in cp.x, expecting atoms with USPP listed before those with NCPP, which in turn are expected before local PP's (if any). Otherwise you can mix and match, as long as the XC functional used in the generation of the PP is the same for all PPs. Note that it is the hardest atom that determines the cutoff.
First, a general rule: when you ask for a pseudopotential, you should always specify which kind of PP you need (NCPP, USPP PAW, full- or scalar-relativistic, for which XC functional, and for many elements, with how many electrons in valence). If you do not find anything suitable in the ``pseudo'' page of the web site links, we have bad news for you: you have to produce it by yourself. You can use the atomic code: have a look first at the contents of the library of input data in atomic_doc/pseudo_gen. Otherwise, you can use any other code producing a file format that is either recognized by QUANTUM ESPRESSO or for which a converter to the UPF format exists. New contributions to the PP table are very appreciated (and very scarce).
Please consider first if DFT is suitable for your system! In many cases, it isn't (at least ``plain'' DFT: GGA and the like). If you are still convinced that it is, see above.
What is available (no warranty) is in directory upftools/. You are most welcome to contribute a new converter.