Which type of antenna may I use with XBee-PRO 900HP RPSMA? May I use 2.4 GHz antenna or only 900 MHz?

Antennas are tuned for specific frequencies. While using an antenna that is not specific for that frequency may not damaged the radio, it is always best and you will get the best performance from an antenna that is designed for that specific frequency. In this case, 900 MHz.

Just for a test may I use a 2.4 GHz antenna?

I would advise against it.