A Bristlenose catfish would be fine, only grows to 6" and is relatively inexpensive.
There are plenty of Plecs that stay small but many of the ornate ones retail for £20+.
The Tetra would be better in a shoal of 6.
If you rehome your current Plecos, you should be fine with one BN provided you keep up with regular water changes and maintenace.
I'd stop using that particular lfs as they are talking rubbish.
The L number was given to Plecs to help identify them as their were so many types. Once the fish had been identified and given a proper name, the L number was no longer required.
Most people stiil refer to them (including myself) by L number because it makes it easier for other people to know what Plec they are talking about.