Generally, yes -  Providing that it is FM and reasonably modern.  You must use the appropriate manufacturers crystal in the correct units. e.g.. MPX crystal in an MPX transmitter & Futaba crystal in the Futaba receiver. 

What is the difference between Vario and full radio sets?

The Vario set is designed for modellers who are either trading up from other Multiplex or other manufacturers sets, who have other servos etc.  They are simply the transmitter with battery and receiver.  No crystals or servos are supplied with the Vario'99 sets.  The full sets contain everything needed to run the set.  All you need to buy is your choice of charger.

What is the difference between PCM, PPM & IPD Receivers?
Check this out here.

Is Multiplex really HiTec?

No.  HiTec is owned by Mr. Chun Park.  He also happens to Multiplex as well. They are two separate companies.  While a certain amount of cross fertilization takes place, Multiplex is not HiTec.

Can I use my JR and Futaba servos on Multiplex receivers?

Yes.  Multiplex UNI plug receivers  will fit all current major manufacturers servo plugs.

Can I open my transmitter without infringing the warranty?

Yes.  Multiplex transmitters are designed to be opened by the user.  The have simple clips to enable quick access to the inside for alteration etc.

Can I alter my transmitters configuration?

Yes.  Multiplex transmitters are designed to be added to, using extra switches and channels.  You can also rearrange the switch or even channel positioning to suit you own spec.