To can-open is to kill a stack so that your ally can blitz through and attack what is behind. One example, If the german and italians are in rostov, there is two paths to moscow. If the main russian army is in Bryansk and the russian has 2 infs in Tambov. Then the italians can use 2 mechs, 1 tank to try to kill those two infs. If they die, then all of the german mechs can take moscow and get the ussr income. Germany has to be careful when doing this, so that they dont lose their entire army by splitting it.
That is the general idea of can-opening. So, What units to use? Answer: Any unit you can get there. And this is why most canopening forces are mechs+tanks. However, there is one trick the axis can use. Germany can take Yugoslavia on G1, then italy can do a noncobat move into Bulgaria. This way you have 4 extra landunits for canopening. In this way it is very easy for italy to get a force where they have 4 inf, 2 mech 2 tanks and 2+ planes.