Mixing Betta's and Gourami's isn't recommended, although depending on the fish it can work sometimes. I wouldn't do it unless you have another tank to put one in if it doesn't work.
I personally wouldn't do a pearl in that size tank anyways. JMO.
But I think it would really depend on their personalities, probably more so the betta. As mentioned above I would definitely have a backup plan in case things go wrong.