More Fish?... Or Not More Fish?... That Is The Question

So after the horrors of moving more than a year ago, I now have a happy heavily planted 50+ L (have subtracted the substrate volume etc already) tank containing an internal filter and an extra bubble filter with:
3 dwarf rainbows (getting another at least, but they seem to be hard to find atm and the one shop that sells them I don't want to get them from since I've ended up with bad cases of disease every time (my water stats were perfect before you ask )
3 amano shrimp (funny little critters)
1 dwarf pecoltia (or however you spell it lol)
and 2 coolie loaches

All fish are happily settled and seem to get on well, none of them are shy, and even the pecoltia and coolies will wriggle around during the day in the open areas. (I've even seen the coolies steal food from the rainbows, the poor rainbows can't spot it on the substrate since it blends in so well although the rainbows may be playing the long game, letting the coolies find it for them so they can get it off the substrate again... if the shrimp don't steal it first lol)
The rainbows (1 male, 2 female) are real beggars for food and will eagerly swim up to wherever I am sitting, the side if I'm at my desk or the front if I'm watching 'fish tv'. However after I've dropped the food I'm not interesting anymore . Although there is a little bit of chasing between them occasionally, mostly they seem to be happy and do their little shimmy dances and seem to be playing 'hide and seek' in turns (or they are simply nosey and want to see what the others are doing lol).
My pecoltia has abandoned his coconut cave in favour of the more open 'log' and will happily bulldoze about (poor plants, although he does try to be careful I think, it's proving a challenge to establish new plants since he keeps tearing them out when he wriggles past A lot are floating at the top, but they are still growing. Currently using a moss ball to cover part of them until they get established, then plant another one but it's slow going since I only have one. All in all I have about 8 different species in various stages of settlement).

So... I was wondering... Would it be wise to add fish to what is an essentially happy tank? And if so does anyone have any ideas of what? and what a good shop in the Bristol are is?
Since there is an unclaimed coconut shell (home made ), could I keep something like a pair of rams? I've had kribensis before, however they started breeding and went on a killing spree (RIP froggies and corries) while I was away for the Christmas holidays (typical).
I've also had betas, but I don't think they would work with the (potentially) nippy rainbows.

Any ideas are welcome

There are two good shops in Bristol I liked and could get to - one in Bedminster called The Aquatic Store (they're on Facebook and post stock lists etc fairly often). The shop didn't use to look much but they had interesting fish and the shop keepers know their stuff. The other is the Maidenhead Aquatics between Brislington and Keynsham, I think the garden centre it's in is called Cadbury. Those were my shops of choice anyway!!