I use an old Traynor 4x10 with 16 ohm EV speakers along with three 2x15 cabs all wired for 16 ohms. That gives me a 4 ohm load that my modified 1200S (tube type) is happy with. For smaller gigs, I drop off one or two of the 2x15 cabs. For really small gigs, I go with a smaller amp and a MusicMan 115RH with an EV. I haven't tried any of the 4x10 cabs you mentioned, you're going to have to pack your rig down to a store and try them to see what works for you.
The 300T should be set for a 2 ohm load. The impedance of the 4x10 will determine (to some extent) the balance between the two cabs. If it's 4 ohms, it will draw the same amount of power as the 215S. If it's 8 ohms, it will only draw half as much as the 215S, but there is a lot more going on than the simple parallel resistance equasion suggests. When dissimilar cabinets are connected in parallel, the different resonant frequencies cause a complex interaction that may or may not sound good. You could end up with some notes being noticeably louder or softer than others or notes that "fart out". You're just going to have to try it.
A 4 ohm cabinet and an 8 ohm cabinet in parallel result in a 2.66 ohm load. Your amp will probably produce about 250 watts into that load when set to the 2 ohm tap. You might not even notice the drop in power and, it won't hurt the amp.
The equasion is: Rtotal = 1/((1/R1)+(1/R2)) or Rtotal = (R1*R2)/(R1+R2)
