Corys do best on sand, but if your gravel is small & rounded, not sharp, they shouldn't damage their barbels.
They do like to be in larger groups, 6 is better
I had gravel in my tank when i added my corys but the didnt seem happy on it and stayed away from the surface. I replaced it with JBL Manado and they seem over the moon and spend loads of them shift across the surface
5 is usually recommended as the minimum. I think the bio-load of your tank should be able to support that
Sand is a must with Cory's, an absolute must, as if they rub their barbles away they can get infected, I lost two cory's that way. Also, any lost barbles won;t always grow back
Also a cave they can get away into, my Panda's love to settle down away from light and other fish and just rest. (Not a must but mine sure do enjoy some time to themselves in the cave on occasion )
thinking of switcing to sand but seems like a hassle and dont want to lose any fish in the process, i have a spare 20l tankthat I could put them in while I make the change but would I have to keep some of the gravel in the 60l tank in bags for a while, while the bacteria grows abit?