Any fury or human can get fleas (different strains like different hosts) and cavaliers certainly can. I must admit though, that i've never had a dog get fleas, so don't routinely treat.
Regarding heart worm : if it's in your area you must take preventatve action. Only last year a member of this forum and one on another list, both reported losing pupies to this and they are in the UK where it is very rare.
Spaywise. Talk to your vet. Some say that you should allow some maturity before, but others say that spay before a first season reduces the chance of mammary tumour to almost zero, after one season, reduces the chances rather less and after two makes hardly any difference. All spaying, of course prevents pyometra.
Barbara, Monty, Joly and Teddy.