They need to match our platform.
2) Some types of molecules involved with life have chirality so the chances are small that our biological components as a whole would be compatible. (Think of it as several different kinds of "endianess." There would be a coin-flip for each molecule type with chirality.) So at worst, if we ate each other, we'd give each other molecular "indigestion."
More complicated operations, like that of viruses hijacking our cells to replicate themselves are probably right out. Invading foreign cells couldn't get very far in our bodies if all of their enzymes malfunction and they find our material "indigestible."