can a bacterial protease inhibitor cocktail be used for Western Blotting involving HUVEC cells and HT-29 adenocarcinoma cells?
In general, leupeptin (a cysteine, threonin and serine protease) and aprotinin (sometimes called Trasylol; trypsin inhibitor) are included in most mammalian cell lysate buffers. Whether you need them is really up to your protein of interest: if you know how it gets degraded in vitro, then you should be fine with inhibition of just those proteases. If instead you want to preserve as much protein as possible, then you should add them. Should you not use them, then yes, those proteases will degrade some of the cell proteins.

The good news is that you can buy aprotinin and leupeptin individually and supplement them to the necessary concentrations for a working lysis buffer.