Try stuffable nappy covers - that way you can use the terry nappies, but you may need to cut them in half and/or just fold them ONCE in half (or into thirds, as I do with my flats) and stuff them in. That way you're just buying a cover/fleece liner that's sewn together and not a whole new nappy system. I have twice as many flats as covers and that's GREAT for the drying: the stuffables dry really quickly but the nappies don't so there's no panic over drying.

Not sure about sizes, sorry, the ones I have come in 2 sizes and DS has been in size 2 since about 8m I think. They adjust well for his tiny waist and tiny legs - he's still on the second popper in age two-and-quite-a-bit! So my skinny boy is well in them and there's loads of room to "grow". But just buying covers should be cheaper than buying a whole new system so two sizes shouldn't be so bad.

HTH.