hi lukitoh, there would be no problem, as it is no different to a guitar body pack, i dont know if you can get them in stereo, but if so that would be the way to go with a keyboard, and both keyboards and guitars have line-level signals..bear in mind though that these are audio only, there are some new midi wireless transmitters but they are still pretty expensive..as for the mics, wireless mics have been used for years now so there would be no issue there either...one thing to be aware of though is that some of the cheaper wireles rigs tend to use a fixed frequency, so just make sure you have no duplicates and off you go
hope that helps,
dennis
PS that information is solid for australia where it is all UHF, but in the states i think vhf is still the band of choice, so it would be worth double checking, BUT if you specify a UHF receiver/transmitter setup you shouldn't have any problems..