What size tank is it? That's the biggest limiting factor here. If those are accurate measurements (inches and not mm, I'm assuming?), your options are limited as far as genuine 'monsters' go.
Watch your pleco if that's what you mean by 'sucker fish,' lots of people including myself have had issues with plecos sucking the slime coats off of Polys.
As for a test kit, the api master kit has everything. There are better kits, but the api kit is affordable. Avoid test strips--they aren't accurate.
Do you know what kind of Polypterus' you have?