Eventually you'll want to forge all of the gear that you use up to the level cap of 65. Any gear with a level requirement below 40 is just a stopgap until you can get something better, with the exception of a handful of event items. You may need to forge up such stopgap gear to make yourself strong enough to go get the gear that you actually want, but don't waste too much copper on it. For temporary gear, I generally didn't make its forging level go above the general level requirement.
You want to get your main team to level 55 as quickly as you can. Once that is done, then you can spread stuff out and level other generals to 55. If you fuse a general who has been leveled up a lot into another general, you get about 70% of the experience back, so it's not wasted. At this point, I mostly level up various generals just as a way to use the experience cards, and now have 80 level 55 generals.
