Yes they can.
However, it will increase the chances of chemical warfare (aleopathy) between corals. Certain corals like toadstool leathers (sarcophyton) and finger leathers (sinularia) are renown for producing chemicals that are determintal to SPS. Certain LPS corals are well known for extending large feeder tentacles at night that sting other corals. A lot of these problems can be dealt with by placing corals properly in your tank and running activated carbon (changing it frequently).
I have over 30 species of corals in my 75g mixed reef -- leathers, sps, lps, mushrooms, zoathids -- and all are doing well. Many TOTM's here on RC also have a wide variety, although most tanks that are SPS dominated will have a minimum number of softies.
HTH,
Mariner