5 gallon secondary for 2.5 gallon batch of hefe?

hi. i am a noob, with this being only my 3rd time brewing. I am brewing a hefe for 2.5 gallons for the first time. all my other times I have brewed 5 gallons. I know using a 5 gallon primary bucket for a 2.5 gallon batch is ok because of the co2 pushing out the oxygen. but what about a 5 gallon secondary?? the only reason i am fermenting using a secondary is to add fruit to my hefe. if i wasnt adding fruit then i would leave it in primary for double the time. if it is bad to use such a big secondary, is there anyway i can add fruit without secondary fermentation, and get the same effect as if i did add it in secondary. such as adding the fruit juice to primary once the fermenting slows down?? right now i dont have the extra cash to buy a 3 gal carboy, but will be getting one very soon. thanks a lot!

I know I'll get flak for this, but if it was me, I'd get 3 gallons of water and split it into the three jugs.

However, I already have 3 airlocks and stoppers, so if you don't that may be a problem....

I can't say how to get around not using a secondary because I'm not sure how the results would turn out, but when you add the fruit, that'll give a little bit of sugar for the yeast to eat and produce a little more co2, so you might be okay.