Hybrid layers aren't large hens and they are usually much smaller and leaner than pure breeds.
Personally I wouldn't mix pure breeds and exbats though I'm sure there will be a lot on soon who will have done this with no problems who will disagree.
Batts are vaccinated against lots of infections which your pure breeds won't have been. If live vaccine has been used (which is cheaper and quick acting), and you won't know and no one will be able to tell you, they could be carriers of diseases to which they are immune. That's a big risk to take with your pure breeds. The chances may be low but the consequences could be dire so why risk it. Also your pure breeds could be a risk to the batts if they were shed kept. This is a case where your head must rule your heart.
Despite the risk if you decide to get some exbatts get free rangers ONLY and at least they won't be entirely alien to your bantams. I've asked people who do this ex batt rehoming and the ones I've asked have been completely ignorant of these issues which I find worrying in the extreme. You should ask the charity who you are thinking of using what vaccines these birds have had and if they are all non live vaccines and therefore safe and if they can't tell you don't take the risk.
If you do get them check each bird carefully for any signs of illness, runny noses, sneezing, dirty rear ends, depression and particularly rattlely breathing. If any show signs reject the lot. If they are free rangers worm them and check all of them for parasites, lice and feather mites. Quarentine them for at least three weeks just to be sure they aren't about to develop anything nasty but even so it won't show up if they are infection carriers for diseases they are immune to.
HF