You might want to attenuate the output of the synth just to be on the safe side. My old Kawai had two output jacks; one for -10dB, and one for +4dB. Erring on the side of caution I always used the -10dB one, and kept the volume quite low on the synth.
The signal voltage is different from the power supply voltage though, too much PSU voltage and you will definitely, 100% guaranteed fry at least a part of the pedal. Too much signal voltage (going down the jack lead from the synth to the pedal) won't damage anything, but will just sound sh*t. Or not, depends on what sort of sound you're after. Most FX pedals have a high input impedance (typically 1MΩ), so there are unlikely to be any problems on that score too.
Using an SVT as a keyboard amp shouldn't be an issue - bass amps tend to make great keyboard amps because they (and their associated speaker cabinets) are designed to handle the low frequencies put out by basses. You may experience a little bit of the extreme top end missing, but a cabinet with a tweeter will be your best friend here.
Using a guitar amp as a keyboard amp isn't really recommended. Not because it's dangerous, but because guitar amps are designed with a particular sound in mind, and the tone shaping part of the guitar amps are generally engineered to flatter the fairly dull sound of an electric guitar. Also they can't take the extreme low frequency output of bass guitars (or keyboards), and will distort at much lower volumes. They might also (if you're really lucky) start walking as the entire cab vibrates - good fun!
In short, go for it.