im sure they would get on fine with a platy, but from the other posts of yours that ive seen you have a 60 litre set up - and congo tetra get quite large (about 3 inches or so) so probably wouldnt be suitable in smaller tanks.
im in the process of setting up a nano tank half the size of yours, and theres actually a really good choice of smaller fish - take a look at some of the micro rasbora. i saw some chilli rasbora and green rasbora in the lfs today - they look great - really pretty fish and make a nice tight shoal, if thats the kind of thing that youre after.
nick