Assuming that you can handle the waste they produce, and don't care about them taking over the whole tank, can have as many BTA's (( E Quadricolors )) as you want.
As for mixing them with "carpets" (( should be more specific, they have different care )), I would not to it with an BTA and an S. Haddoni. I tried this in the past (( and so have others, and have had the similar results )), and it didn't work out -- the Haddoni wouldn't stay put until I got rid of all the BTA's.