Bettas don't mix with guppies.
With gouramies it depends. You have a three-spot female right? Well three-spots are known to be bullies but females are better than males. Realy it's down to the individual fish as gourami characters vary greatly. Personaly I'd not get guppies but maybe try some other colorful livebearers like swordtails, platies or even endler's. None of these have the trailing fins but endler's look quite like guppies so you could try those instead of guppies. Mollies are also an option but I'm not their biggest fan and they get rather big (female swords also get big). Realy, you are the only one who can decide as only you know your fish. If you think she'll be friendly and you know you can move the new fish or re-home them if necessary, go for it, otherwise, it isn't worth the risk when you have plenty of alternatives.
In the 7.5 gallon, you could keep platies with a male betta. I have never tried endler's with bettas but that would probably work out as well.
