Do you already know what size tank? How many rummies do you want in it? A large shoal or just say 5-6? Will it be planted or not? What about bogwood?
Many, many fish will do well with rummynose tetras. Gouramies for a start, small plecos (clown plecos, bristlenose, gold nugget), any corydoras catfish, most other tetras, some barbs (the peaceful ones), most smallish rasboras, danios, small loaches like khulies... There is a huge list and it all depends on what you are looking for in this tank and your personal prefferences...
Why are you asking about rummies specificly? Do you already have some? How many?