OK, lets address each part carefully.
my bees are eating fondant like mad at the moment I believe you. Mine are not eating any at all.
should i give them more or would that be a problem If they need it, you must give them more. If they do and you don't, they may starve and die.
how do you gage when they have anouth When they stop using it?
if i over feed will it be a problem getting them started early in the year No, they will get started when they are ready. After starting there may be a problem with brooding space if the box is still crammed with winter stores, so it depends on whether the bees have already used up their own stored winter food or not. 'No space' will inhibit her laying! Think! She needs space for three weeks laying time before those cells, already in use, become available again.
increase the chance that they will swarm That is down to the beekeeper.
I am wanting to go brood and a half this year or doulble brood not sure which yet
when should i put the extra on By the post contents, you may need a second brood chamber (if the single brood box is crammed with stores and little brood) even for a 'less than optimum' strength colony. Only you will know that, from experience and observation.
All the above is guesswork, not knowing the actual state of play, regarding stores, in your hive(s), or colony strength, etc. (not sure whether you have one or ten from your profile!).
You are the beekeeper and you need to work according to some basic beekeeping rules to manage your bees. They will try to do what they want to do, you can easily facilitate these 'wants' but it is much more difficult to make them do something entirely different.
It is far easier to thwart their efforts by removing their basic requirements, but that will not change the bees' plans. Adding an empty (foundation only) second brood box above a not-so-strong colony too early would severely inhibit the colony development by reason of heat. Having restricted brood space is as explained above. Shortage of appropriate stores (carbohydrate and protein) would curtail brooding, even to the point of brood being sacrificed and ejected on occasions. Water is also a definite requirement for brooding, if using honey as the carbohydrate source.
Beekeeping in early spring is all about managing these factors, which could otherwise limit the brood build-up to the prevailing conditions in the hive - that means a reduced build-up rate. How you go about that is by applying basic beekeeping practices. No rocket science; simple observation and action to expedite the necessary conditions to allow/encourage an increase in numbers of workers, by a little (or perhaps a lot), ahead of what they would otherwise achieve.
Now, if you can get them expanded greatly, and early, there may be other tasks for the beekeeper - prevention starving and, later, of swarming.
Regards, RAB
