Yes, no paradise fish with other paradisers or gouramis. Same goes for bettas or any other anabantoids. Though we have a female betta in both of our tanks which house gouramis and all has been well.
Razzies and neons are, in my experience, easy to keep and quite friendly to other fish.
I've had good luck with otos so far. Some of them do succumb to stress from being moved though, so be prepared for the possibility of losing a few. Plus they do better in very well-established tanks. The only places where I've lost any so far are in tanks that are a bit newer.
You could keep some corys. Those guys are hardy and peaceful. Upside down cats are a lot of fun to watch. Yo-Yo Loaches and Kuhli loaches often do well in planted tanks. You could also consider bamboo shrimp, cherry shrimp or Amano shrimp. Just be careful to make sure you don't have any fish that will make them into dinner.
As far as filtration goes, pretty much any filter that is handling a decent sized aquarium is going to be three stage just by default. That is to say, you will have mechanical media and likely chemical media, and the biological filtration will grow on all filter surfaces anyway. Though what is meant by a "three stage" filter is that there is an additional area meant solely for growing bacteria. (Bio-Wheels, Bio-Balls, ceramic rings, lava stone, etc.)
Filters seem to be a matter of personal preference, so instead of giving advice to make myself feel important I'll just tell you that pretty much any decent quality filter that is flow rated to a little more than what you need will do as good a job as any other filter. Filters are rated in gallons per hour, and with that is determined the size aquarium it can effectively filter. For example, most filters that are rated at 150 gallons per hour are said to be suitable for tanks up to 30 gallons. But it's always wise to supercede that number as much as is realistically possible. I would therefore choose a filter that is rated maybe 200 or more gallons per hour. Which one you choose will depend more on your specific needs and tastes. But don't go too crazy, or you'll find your fish being tossed around in the currents of a filter too powerful for your size tank.
Marineland's Penguins/Emperors and Hagen's Aquaclears are popular, well made hang on back filters that will give you efficient three stage filtration. Both make quality 150 and 200 gph units. I hear mixed reviews about the Tetra Whispers.
If you're considering a canister filter, which is viable an option once you're going for a tank that's 20+ gallons, then you have the Rena Filstars and The Hagen Fluvals as your good bet in the consumer level units. Of course there will be people on here that will insist you buy an Eheim. Now while Eheims are indeed a better quality product altogether, they come at a pretty steep price tag which I find questionable versus their actual performance. I think the argument for Eheim vs. Filstar or Fluval is more in build quality when they're being used on a general aquarium. Then again Eheims come with a one year warranty whereas Filstars come with a two year warranty. Something to consider.
Low light beginner plants - I did well with java ferns, java moss and cryptocorynes. You might also try anacharis, hornwort and anubias.
