I'm going to buy a loop station (rc-300) and a cheap electric violin. I have an amplifier but what I don't know is, are all electric violins connectable on my amplifier?
Yes. In fact this is specifically what they are designed for. You can hook them up to anything you'd use for an electric guitar if you want the sort of tonal colour that gives, or you can use a clean PA if you want an unaffected tone to come through.
The whole point of an electric violin over a standard acoustic (normal) one is that it has the facility to plug a lead in, from it. To an amp., a p.a., a mixing desk - and - a looper pedal!