Hi Jet.
When an all-tube amp is rated at 20 watts it refers to clean power, and it pumps out that amount of power at around 4-6 on the master volume (right before it starts to break up). When maxed out it can easily pump out 160-180% of the power it's rated at.
If you plan to dime the amp all the time, you should let your amp see a total load of roughly twice the wattage of your amp. Ergo 40 watts (or if you're using a 2x12 cab, you need at least 20 watts per speaker). For the ohmage just make sure the amp out matches the speaker load and you're golden. There are tonal differences when you do a mismatch (less headroom, earlier breakup, lower volume) but these are minute differences and you're taxing your output transformer needlessly if you don't know what you're doing.
That's not to say that you can't use a 25watt speaker for your 20 watt amp. (my AC15 clone uses a 25w alnico speaker). But if you do I suggest you watch your volume knob, and be sure to check if anything is heating up out of the ordinary.
For speaker wattages you can go as high as you want but the rule of thumb is the higher the wattage the heavier the doping and darker the base tone (with exceptions of course). There is also less cone cry which you may or may not like.
