As the akg wireless standard reciever is too large to carry with me, I was wondering if I could use a reciever from another brand, for example, sennheiser???
I wanted to buy PR40 diversity, but looks like it is discounted and not available any more. So I have to find a solution.
If the frequencies are the same akg transmitter and, lets say, sennheiser reciever should work together, or am I wrong...?

Generally speaking, transmitters and receivers from different manufacturers (or even different product lines in the same manufacturer) can NOT be assumed to be compatible unless you do some extensive research and/or experimentation to prove otherwise. There are a dozen factors that influence how audio is encoded or modulated (and then decoded/demodulated at the receiver end).

Used equipment is generally available if you are diligent about searching/waiting for it to show up on eBay or Craig's List, etc.

No - you need to keep radio systems in sets.

Different manufacturers use different noise reduction systems that are not compatible with each other.