Impedance mismatches are a lot more of an issue for solid state amps than for tube amps too. They will tend to shut down, especially if the speaker impedance is lower than what the amp wants. So a 16 ohm cab might work, but a 4 ohm cab could well blow up the amp. And blowing it by using a cab of the wrong impedance could also void your warranty, so make sure whether that's the case before proceeding.

Good call on keeping the 5150 around! The SpiderIII might be useful for some things but the 5150 is great at what it does!