Humping and chasing is fine. As above they are establishing a hierarchy which is perfectly normal. Frankie didn't leave fidget alone when i first put them together but eventually Fidget submitted and Frankie became dominant and that was that, best of friends ever since!
Are they actually fighting or just chasing? A bit of fur pulling is ok but if there are any wounds from biting then perhaps it is better to separate - ONLY for a few minutes until they calm down and try again.