I turn them down all the time. A placement has to be right for you. There is a big shortage of foster carers so it doesnt make sense that you would be discriminated against going forward. Act in haste, repent at leisure. The wrong placement will be miserable for you and your fmily and for the child too.
Agree with pp. Just explain to your social worker why you don't think this placement would work and as long as you're being reasonable there's no reason to think you wouldn't be offered another (more suitable) placement another time