filter

If you want to change the media in your filter do it in stages. The mechanical media can probably just be rinsed clean in old tank water. The carbon if you use it can be changed every month or in my case every 3 months. The biomedia should never be changed. The knowledge you received about vacuuming the gravel is absolute nonsense. You should vacuum it at least every 2 weeks when you do a waterchange starting from day one. Vaccuuming the gravel contrary to the myth will not do enough damage to the biocolony on there to not warrant it. Any damage done will be fixed within a few hours as the bacteria quickly repopulate it.

much depends on the status of your biofilter. if you've got zero ammonia and nitrite now (a 'cycled' filter) you can change "some" of the media. never replace all of it at the same time or you'll eliminate your bacterial colony. if you have sponge media such as in an Aquaclear, you can rinse it as needed without worrying about your biocolony .. it's virtually impossible to rinse out all the bacteria. i rinse my Aquaclear sponges every month.

if you've got filter "cartridges" with polymedia on both sides and carbon in the pouch within the two halves, you can cut some of the pad off the filter and place it inside or in back of a new filter and thus, seed it with alot of bacteria.

as far as vacuuming the gravel, i do so at every water change and i do a 50 percent water change on a weekly basis.

What type of canister is it. You can usually tell if the mechanical media is reusable, usually the fine pads need replaced, course pads can be rinsed and reused. The bio media should be rinsed in tank water at your maitenence intervals as well. What I mean by that is when you break the canister down for cleaning.