I've recently bought a cheap spanish guitar so's I can learn, and had it restringed by a mate so I can play it. Can you do this with most other guitars, or will it sound awful? (I'm a rookie). Also how difficult are other stringed intruments (banjo, ukelele etc.) to play in comparrison.

there are a few issues you might run into when flipping the strings on an instrument like that... nothing really major though...

first off you might get some slippage with the high E in the nut at the end of the neck since the groove is made to accomodate the larger low E string... flipping the nut usually fixes that problem...

another problem you might run into with acoustics is the pickguard being on the wrong side so depending the type of pick you use and how much emphasis you put into your strumming you might end up scratching up and wearing down the side of the body by the soundhole (kinda like willie nelson's guitar)

and the main problem with flipping the strings on an electric is the fact that all the knobs will be under your arm while you're strumming... the angle of the pickups is reversed too (gives a smoother low end and brighter high end)

as far as other instruments go you can usually switch the strings so long as they all meet up at the end of the neck... 4 string banjo would be fine... 5 string wouldn't work since one of the strings winds half way up the neck...