Starter Community 70 Litre Warm Freshwater.

i am going to set up a 70 liter heated freshwater tank and  am looking for suggestions re a decent size colorful centerpiece (possibly a pair). two groups of about 4 smaller, but pretty species, and maybe a couple of shrimps.
suggestions welcome.
 

The problem with having two groups of 4 smaller fish is that small fish generally tend to be shoaling species which need at least 6 of the same species to feel safe. They live in groups of hundreds if not thousands in the wild and if there aren't enough of them, their instincts tell them something has eaten the rest of the shoal so they'd better be on the look out for the predator or they'll be next. This leads to the fish becoming very stressed, and stressed fish get sick easily.

In 70 litres you should be able to fit 2 shoals of 6 or even 8 fish each, provided you choose species that don't grow more than ~3cm.
If you want brightly coloured fish, look at ember tetras. They don't look much in the photo in the profile on here, or in the shops, but when they settle in your tank, they are bright red. If you look at the red fish at the top left of the photo here, these are my ember tetras.
Other bright tetras include neon/cardinal tetras - 2 related species but with small differences.

However, I would not get these fish if you have very hard water as both prefer soft water.
For hard water, look at livebearers - platies come in bright colours, as do the smaller enders, though males only with livebearers or you'll be over run with fry.

Soft water centrepiece fish - the laready mentioned dwarf gourami or honey gouramis. Although the female honey gourami is greyish beige in the natural form, there are two colours developed by selective breeding which are more common, and the females are the same as the males - yellow and red (also known as red robins). There's one of my yellow honeys in that photo.


If you got smallish fish, you could try a group of cherry shrimp, which are bright red. But at least half a dozen of them, not just two. With any shrimp you would need somewhere for them to hide when they moult as they are easy prey for any fish until their new skin hardens. Some sort of cave, and lots of plants, real or synthetic.

You need to cycle the tank first, then add all the fish straight after the cycle finishes. If you don't know what I'm talking about, read this

You can cycle with fish in the tank but it is a lot of hard work. Depending on just how many fish are in the tank, it can mean several water changes a day for a few weeks to keep the fish from being poisoned until enough bacteria have grown in the filter. And I do mean weeks, possibly even months.
If you do choose to do a fish-in cycle, guppies are not a good fish to use. They are very inbred with breeders using some not very healthy fish to get the appearance they want, and this has led to guppies being very weak fish that commonly die for no apparent reason. Guppies are unlikely to survive the process of fish-in cycling. Neither are neon or cardinal tetras.

For bottom feeders, I prefer sand*. They can push their heads into it with no problems. Small rounded gravel is fine as well. Avoid sharp gravel as this can cut the mouths of bottom feeders. Gravel should be kept well cleaned if you get fish like cories as decomposing food, fish poo etc can encourage bacteria which can infect bottom feeders, particularly on sharp gravel as the cuts allow the bugs to get in easier.
As for colour, a lot of people say that dark colours bring out the fish's colours better, but I have pale sand in my tanks.

Plants - I'll leave those the plant experts.

* I know they aren't what people think of as bottom feeders, but a few years ago I had a pair of Bolivian rams and small gravel on the tank floor. The latin name of rams of both species means small earth eater - they feed by taking a mouthful of substrate, sifting the food out them spitting the substrate back out. The female ram got a piece of gravel stuck in her throat. She did finally get it out after several days, though I don't know which direction it went, up or down. She was never the same again, and died a couple of months later. That's when I changed all my tanks to sand.