Your not that far away from Manchester then... which is stuffed full of good shops, well that and the surrounding areas. Pier Aquatics and Rare Aquatics are two of my favorite shops in the country and stock pretty much all of the fish we both like so much
Both are probably about an hour or so away?
Breeding fish is always a risk in any tank, personally I try to stay away from it with the largest fish in the tank - for example in my six foot the only pair I have is a pair of Bujuqueena sp. who should be manageable in the tank given their small adult size of about 3 - 4 inches compared to my 9 inch mouth brooding severum and 8 inch threadfin acara. In hindsight maybe I would recommend going for 2 males - though go for the Sevs in this case - get two youngsters, but not so young you cant see the pointed dorsal fin and face squiggles across the nose
And grow them on, they should co - exist but there is always a risk.
Acaras - its a up to you I added these in just out of my preference but there are a bunch of other options out there
You could try the Threadfins but they are very hard to sex when young and two males will struggle to coincide with each other as they can be quite boisterous with each other to say the least. You will find the Threadfin will join in with the hierachy of the Geos in the tank, which is quite a natural relationship in the wild - though I am not sure which species they cross paths with in the wild.
Interestingly Threafins are hard to breed in aquariums, they build quite large caves and excavate a large area before laying eggs deep inside - it can happen in tanks but its quite rare.
Tetras - I suggested the Lemons as I like the understated nature of them especially against the flashy fish in the tank - Bleeding Hearts would work nice though
Emperors might work but you would need to find quite large ones to start with I think. Any of the disk shape tetras will work though, just avoid the torpedo shape ones like Rummy Nose, Cardinal etc.
There are probably other catfish species around that would work it just depends on what you like planetcatfish.com is a great resource
Funnily enough I find Flagtails hard to find, I have 3 and all of them have been the last one in 3 different shops, they all look different as well which is funny
Plecs - I know what you mean, some commons are pretty nice, sailfins are just as common to be fair - a big black plec with white spots sounds like an Adonis... which is a huge huge fish growing to like 3 - 4 foot
My tanks is a 6 foot long 135 gallon so not quite as big as yours. I had quite a big disease outbreak last year and then moved house so left the tank a little light until recently so some of the fish in there are quite young. I have 2 FX5 filters on it as well.
1 x Mouth Brooding Severum
1 x Threadfin Acara
2 x Bujuqueena sp. Gold Stripe
6 x Cupid Cichlids
5 x Annostomus Ternetzi
6 x "Giant" Tetras
2 x Agenisous sp.Atronasus
4 x Dwarf Hoplo Catfish
3 x Flagtail Catfish
3 x Whiptail Catfish - 1 x Loricara sp, 1 x Sturisoma Aurem, 1 x Sturisoma Barbatum
Wills