For Guppies I like a 60 x 30 x 30 or 60 x 30 x 40 tank 54L / 60L which I will split either 1/3 and 2/3 or 1/3 , 1/3, 1/3 with tank dividers.
Depending on what space you have then this is a good size to tuck away, if you are patient and look around you can sometimes pickup the above complete setup even with a stand very cheaply second hand, bit of a clean up and they look like new.
Shop around for the dividers I use the penn plax ones generally and have seen the same divider at £3.99 to £10.99 (18inch high to cut for 15 inch high tank) in different LFS.
StatMan