Probably yes, depending on which fish and on water parameters. A very rough guide for initial stock is 2.5 cms of fish length at adult size per 4.5 litres of water. Your 260 litre tank will be about 230 litres after gravel etc, so (230/4.5)*2.5= 128cms fish initially (added over a few weeks). If you increase the shoals to 6 Albino/Bronze Cories at 7cms each and at least 6 (preferably more) Pygmy Cories at 3cms each that makes (6*7)+(6*3)= 60cms of fish. Then there's your Plec - how big is he and what sort?
Once the tank is mature, the total can be increased again to a max of about 200cms of fish if nitrates are well-controlled.
Hmm, well the Common if it's like mine will get absolutely enormous. I got her when she was about 4-5 inches and was stunted from being under-fed and not well cared for and 2 years later she's about 16 inches long including her tail. That's why I have to save up to get a 6ft tank next year! A pic would help as there are various types of common plec - mine is full size now (she's a Pterygoplichthys pardalis), but some of them can easilt get to 2ft ong so it's important to know which you have.
The one you describe as a dwarf could be one of many - any changce of a pic o hime so we can work out what he is? It's quite important to know apart from potential size as there are hundreds of diferent small plecs and they need quite different foods. Some are vegetarian and eat algae and vegetables, some are carnivores and nee things like bloodworm, prawns, cockles etc and some actually need to eat wood.
The dwarf has been under his driftwood all morning so no chance of a pic until tonight prob, The common he is almost impossible to get a pic of but if i get a chance i will, The common will most likely have to go back with my dad when he gets his 5 foot.
From the pattern on him I'm leaning towards Pterygoplichthys gibbiceps which can grow up to 18 inches long (45cms), but without seeing the dorsal fin it's hard to be sure. Here's a profile with some pics of that species which might help you confirm if that's what he is.
No, he'll need similar to mine which will be 500-600 litres. The tank I'm getting to house my plec and Clown Loaches is 6ft x 2ft x 2ft. She's in a 5ft tank now and definitely could do with more space.