You can certainly pull tubes on a triple. It will affect the load that the amp wants to see - pull one pair, and the jacks will want to see loads of 6, 12, and 24 ohms instead of 4, 8, and 16, respectively. Pull two pairs, and it becomes 12, 24, and 48 ohms.
You could run either a 4- or 8-ohm load on the '6-ohm' tap. I'd probably only run an 8-ohm load on the '12-ohm' tap, and a 16-ohm load on the '24-ohm' tap. The '48-ohm' situation is, of course, fairly useless.
Contrary to what appears to be nearly universal opinion, you don't have to pull a rectifier tube. In fact, on a Dual, you should NOT pull one, unless you take a very specific precaution.
If you don't pull a rectifier, the amp will feel more like it's SS rectified. In fact, it'll feel that way to a degree anyway, since by pulling power tubes you've lightened the load on the power supply and it'll sag less under any circumstance.
However, there's a parameter related to rectifier tubes that I've never seen mentioned in these threads, and that is maximum input capacitance. When used in a capacitor-input circuit (which includes 99.9% of all guitar amps), there is a maximum amount of capacitance that tube rectifiers are rated to 'see'. This is because at the instant you take the amp off standby, the first filter cap looks like a dead short to the tube, and it is asked to provide an infinite amount of current for an instant. The time over which that stress occurs is affected by the size of the first filter cap (or caps).
For the 5U4 family, that maximum size is 40uF.
The Rectifier amps use a pair of 220uF caps in series, for an effective input capacitance of 110uF.
It's clear that the pair of 5U4s wants to see around 80uF (the current capacity is additive, when rectifiers are connected in parallel), so the stock Dual circuit is already stressing them somewhat. The stock Triple circuit, with 120uF of 'capacity', is fine.
So, if you pull one rectifier from a triple, you're showing the remaining pair of tubes a load that's higher than they'd like to see, but not critically so.
However, if you pull a rectifier from a Dual, you're showing the remaining tube almost three times the maximum load for which it was designed.
Happily, there is a way around this - power the amp up set to use the SS diodes, then after charging the filters (i.e. taking the amp off standby), switch to the tube(s). This will let the caps charge from the SS rectification (which effectively has no limit on input capacitance), and then the tube(s) can take over afterward.
