I'm newish to this hobby too and I believe I've learned a valuable lesson along the way.
Like many folk (presumption only) I liked the look of so many fish and didn't really think things through before purchasing them. I was the little boy in a sweet shop. If I saw it and it looked nice I bought a pair. I lost many for no apparent reason but can probably put a good number of losses down to poor water management or wrong water parameters. I'm now at 4 tanks all with a different set-up. I have a small 28 ltr tank for keeping my Krib young fish until they get large enough to sell them to my local aquarist shop. They take a good number of months though so are really just taking space. In fact no sooner had I removed them from their parents as fry within 10 days of hatching, the parents went right into breeding mode again and I found another cloud of fry several days later. I had no room for them so I took the parents out for 10 minutes until the other tank inhabitants had feasted on them.
So I guess that's lesson 1. If you want to breed them be well prepared and have somewhere you can offload the fish when they are big enough.
If you don't want to breed, some fish even in pairs aren't much of a problem anyway. Rummynose for instance aren't renowned for breeding in tank and need a high temperature of around 84F to give them the impetus.
One of the things I have found is that many fish are pretty tolerant of others until they are squeezed in with others of similar size. They do become territorial and it's not uncommon to lose a fish for this reason. The easiest way to prevent it is to hold back on your stock. Too many may look nice. You'll have a lively tank but the fish will be stressed. Best go for few. Either a couple of largish fish with a shoal of things like the rummynose or smallish tetras such as cardinals. I have had little success with neon tetras. They have mostly developed lumpy mouths and gill areas and had to be euthanased. Whether this was Neon Tetra Disease or not, I really don't know but the usual medication didn't do any anything. I still have 10 of them though and they have grown very well to around the size of the Rummynose's but have got traces of white on their tails that doesn't appear to have got any worse recently. Best to avoid though as the other small tetras are just as nice,.
So, I have one tank 120lts holding a pair of firemouth with a mix of other fish, mostly remnants of my over enthusiastic forays into aquarist shops. The male and female Krib are at one end of the tank and the firemouths are at the other most of the time where they sift through the substrate and blow it out repeatedly. They are showing breeding signs but the males of the krib and firemouth have regular full blown face puffed up stand-offs. A 2 year old survivor of a pair of pearly gourami which is now a pretty large fish also takes battle stations with the firemouth male. Then there's a a pair of honey gourami and German rams.
I'd do things very differently now though and do as I suggest in having perhaps the firemouths which are beautiful fish, lively and colourful with the small tetras.
My secfond tank is one I just bought around 3 weeks ago. It's 3' long x a foot by a foot with sliding glass panels on top. Looks very much the sort of tank you'd see in a shop holding stock. I have utilised this deliberately for small fish. I* really do want to breed some fish but not the kribs again. I have a small shoal each of ember tetras, galaxy rasboras (CPD's), danio erythromicron, 2 rummynose, a pair of bronze cory, a hillstream loach, a pair of dwarf gourami recently swapped from my big-fish tank so that they can get some peace for breeding which they are intent on doing. To top this off I have a couple of red claw crabs. These were bought to do the cleanup that the corys and loach didn't do. In fact they are wonderful at it. They are shy, dart out to grab food then scuttle back into the safety of the planted area. They are also fish friendly and the only time I see any aggression is when a fish gets close and they just snap at it to shoo it away more than catch it to eat.
I have planted this tank with a good selection of fine grass and mosses to bring out the breeding behaviour of the tatras and the erythromicron danios.
I am well satisfied with this tank. The number of micro fish is around 35 but as they are so small don't look stressed. They are active fish, always on the go and look way better than say, pearl gouramis or kribs that don't have much in the way of in-tank movement. I can sit and watch these all day long.
My last tank is also another small one set up sepecifically for keeping and hopefull breeding the smallest fish I've ever seen; the lovely chili rasboras. I have 10 of these only bought this last week and the tank is settling in nicely. It's not easy to see much in it though as it's stained by using alder cones. This tank isn't one to sit and watch. It's a breeding project more than anything else.
So, your own ideas? Best to make a decision on whether to go for big fish, and I mean anything that grows to over 3" or to take perhaps a couple of bigger fish such as the pearl gouramis that are really lovely fish and put them with a shoal of tetras. I think the ember tetras would be too small though..
If you really want a lively tank then a mixh of small and micro fish would do the trick. Above all and this is sometimes often overlooked, is that the fish you see in an aquarists shop are young. Some of them will look way too big once grown. I had a shoal of columbian blue tetras that looked lovely and colourful but hey, they had become akin to being small piranha in looks and size within a few months. They are also very fast and scare the hell out of the other fish when they dart around. Same sort of thing with the green tiger barbs I bought. They just got too big and became very unusual in that they stayed in the same place for hours on end all hanging with their heads down. I also loved the looks of the Odessa Barb. This has got to be one of the nicest fish you'll ever come across but aggressive. They chase other fish right out of the tank. The other regret I had was in the Golden Barbs. I did what they said and bought 8 of them as they like to shoal. Lovely fish, quite placid but come breeding time and it's like a line up for a gang bang. That wasn't the problem though. They weren't aggressive at all and were always on the move. They just got too big again. I traded them in at my local shop. The biggest female was about 8cm but fat. Like a water pig.
When you research what it is you're looking at in a shop before you make any decision, most descriptions give a maximum size as, say, 3cm. What it doesn't say is that they also grow deeper bodied and often wider in proportion too.
Oh, just seen Essjays comment regadring hillstream loaches. The one I have is no problem at all. It latches onto the tank glass and snaffles its way up and down like an algae scraper. It hides under a flat piece of slate, upside down and has never shown any bad tempered bouts with other fish. I actually prefer it to bristlenoses.