yes skin or skinless doesn't matter. However, since your african only around 2" you don't expect them to finish it in 8hrs. The rule of thumb for the cucumber is you have to take it out of the tank after 8 hours. Hope these help, enjoy !!
Personally, I would feed zucchini over cucumber. It's fine to feed but I find the zucchini > cucumber.
Cucumber has little seeds that your fish won't eat that will clog up your filter's intake.
It's got lots of sugars which foul up the water rather quickly.
Fish simply don't enjoy it as much as they do zucchini. I've fed both to my fish without too much issue but the zucchini gets eaten cleaner and faster.