Well... some background info - male dwarf gouramies are the colorful ones you see in shops. Females can sometimes be difficult to get hold of because, being duller, they don't sell as well. Males are territorial towards each other but will not bother other fish. They do fine with each other provided they have room to establish their territory. Dwarfs preffer the upper layers so they need to have room up here to establish territoryy and appreciate some floating cover such as plants. However, it is essential that this covering doesn't stop them from reaching the surface as they breathe air like us and would quite literaly drown. Due to their prefference for the upper layers, they also preffer it that only a few other fish share the upper layers - so they would rather you didn't have some of the danio species, killifish or hatchetfish. This doesn't mean you can't keep them together - just that you need to keep their prefferences in mind. Males will also chase females around and this can sometimes be stressful for her if she has no where to hide.
Now to answer your questions somewhat more directly:
1. Either would be fine. I'd urge you to go for 3 and get a male and 2 females as this means that the male won't have to compete for territory (females aren't territorial) and that any chasing will be split between the two girls so that neither one becomes overly-stressed. On the other hand, 2 males would be fine as long as there's plenty of space for them to establish territories in the upper layers. Some floating plants on opposite sides of the tank (prefferably where there isn't much current) would be much appreciated but not necessary. A pair would also work but make sure the female has places to hide if the male begins to chase her. Floating plants, as well as caves, rooted plants and pretty much any ornament, would serve this purpose effectively. In a 26 gallon, you shouldn't have any problems - regardless of the fish's gender. Don't forget your last option though - you could get 2 females.
2. Guppies and most gouramies get along fine. Dwarfs won't usualy bother other species so they'll do great together with guppies and pretty much any non-gourami/betta (some gourami species won't get along with dwarfs though dwarfs themselves will usualy only harass other dwarfs), non-nippy, non-aggressive fish would also do fine with dwarfs.
. Thanks also for your excellent replies to other posts that helped me easily determine the gender of these little guys.
