7 speed index won't work perfectly. Depending on one's criteria, it can be made to work OK.
Getting decent quality 2nd hand 6 speed indexed shifters, or even friction shifters is a good option. Many old 6 and 7 speed indexed shifters have a friction/indexed switch, so one can choose which mode to use. All the friction ones will work.
Something like this is perfect:
My writing and info on shifter compatibility can be read
here, if it helps.