You can power any wattage speaker with any wattage amp as desired. The only thing is that if you have a 200-watt amp pushing 30 watts worth of speakers, you will only have about 30 watts worth of amplifier output before you start running the risk of burning up your 30-watt speaker.
Watts are watts are watts. Say that with me, " watts are watts...... " Watts is a measurement of energy dissipation. I.E. the conversion of electricity and current flow into heat. Said another way, watts is the amount of heating power dissipated through a resistive element. A tube amp that is rated for 100 watts is the same as a SS state amp rated for 100 watts, at least in terms of the amount of dissipated power they can produce into acoustic output. Even if by chance the tube amp produces 20 watts more actual watts than that of the SS one, the acoustic output from that 20 watts is negligible ( less than 1db of acoustic gain ).
A 200-watt amplifier does not always produce 200 watts. It produces some number between 0 and 200. If you dime it, sure, it may produce all 200 watts worth of output power. It may produce all 200 watts of output power with the volume knob set at 5 of 10? Without measuring the dissipation, there is no way to know. This is why it is wise to use speakers that are rated to handle the dissipation of your amplifier.
Speakers are resistive elements rated to dissipate X amount of wattage. Guitar speakers are shrouded in a bit of mystery because the rating they use is rather generic vs. that of typical pro audio speakers. Pro audio speakers have three ratings, RMS ( continuous rated power ), Program ( the recommended power amp wattage to use ), and Peak ( the wattage at which the speaker will burn up and die in very short order ). Guitar speakers use a single number. Is it RMS, or is it peak? I tend to believe it is their program rating since most guitar amps will fall right around the typical wattage points a guitar speaker comes in. You don't find too many 12" speakers that come in less than a 25-watt rating. If the rating is in fact the program rating, that means a typical 25-watt Greenback is actually able to dissipate up to 50 watts. This gives you a little bit of leeway.
Lastly is the amount of time a speaker is actually dissipating the power they are provided. Most songs are only 3-4 minutes and then you stop playing for a moment. The dynamics of the song and the brief breaks in between them allow the speaker to cool down. This lengthens the amount of time you can abuse the speaker before it burns up. Unless you are in a DOOM Metal band that plays 14-minute long songs at full volume with zero dynamics, you can generally abuse a guitar speaker quite a bit. I am not saying that you can use 100 watts worth of speakers with a 200-watt amplifier, but that you need to be realistic about how you play, the volume level at which you play, and what risk you are willing to afford.
In the Pro Audio world, we generally ( I say we because I am a sound engineer/technician by trade ) try not to power a speaker with any more than 75% of its peak rated power. 90% of the time, with a properly spec'd system, you don't need to go any higher than the speaker's RMS rating. I.E. the objective is to use the lowest practical amount of power as is required to produce the acoustic output you need.
