Hello tabris,
Others might have different opinions.
1. Can I use other instrument (violine, guitar etc) tuner to tune the theremin?
I think the main thing a tuner is good for is finding the first note if no reference sound is available. I think most tuners would give you this single note indication.
2.What is the fastest music that can be played with theremin?
The only thing that slows down the music is the skill of the player and theremin playing can be quite complicated, unlike a basic keyboard.
3. Can I join the club orchestra (or similar thing) with theremin?
Playing live you need to hear a theremin without distraction in order to play it well. Some people can do it but you should develop your skills in private for the first year.
4. Which one is better?
Both appear to be the same thing but one gives you a mic stand and cable for $10 more.
Good Luck and remember to have fun,
Christopher