the betta may pick on it b/c its a similiar species but other then that its a great choice if the tank is at least 10 gallons. they are very peaceful except when keeping two males together and are much better for smallers tanks.
you will need a heater (im sure you dont have one for the betta and guppies)
just get decorations im sure the gourami will outswim the betta
A male betta may start picking on the guppies. I had a male betta in with female guppies once and that was fine. But he would kill any males that I put in there... And he wasn't even my meanest male betta. (Mr. Meanie Pants lived alone - he even killed a snail once.) So watch out for this.
As for the dwarf gourami... It MAY be smaller than the betta to start, but it will get bigger than it. I had one before. Mine was a male. I'd say about 80-90% of the dwarf gouramis you find in an LFS will be male simply because their colours are brighter and therefore they sell better... Having said this, gouramis and bettas are basically from the same family. You could consider them cousins. This may lead to problems. It may not. It all depends on how agressive each individual fish is. My gourami was quite mean and wouldn't tolerate any bettas in his tank. I never tried a male, but him and my female bettas I tried him with didn't get along AT ALL and had to be separated. They flared and fought from the second they were together. After a couple hours of watching them I determined that if I didn't separate them before the night was over I'd either watch my trio of female bettas pick apart my gourami or my gourami would pick the females off one by one.
Now these aren't 'be all, end all' rules. These are my PERSONAL experiences coupled with some things I have read on the matter. Everyone has different experiences. It largely depends on how big the tank is (will the gourami and betta have enough room territory wise) and how aggressive each of the fish are. It could all possibly go perfectly well. I know someone who had a larger tank - 55 gallon I believe - and she house a gourami in there along with a male betta, and several other fish, and everyone lived in harmony. But it was a big tank with lots of plants (ie: hiding places).
Whatever you decide to do, just be prepared that the fish may not get along and have a back up plan - either another tank where either the betta or gourami could go, or make sure that the LFS will take said gourami back (for a refund or in store exchange) if things don't work out as planned.
I had a blue gourami in my tank when I first introduced my rbs. The gourami nipped and chased the p's. I had to find it a new home so they could live in peace.
By the way the gourami was 3+ maybe even 4 inches. The 5 rbp's were only 1 inch.